I am wondering, what endgame, with 3 to 6 pieces, has the longest checkmate with optimal play? Right now, the longest I'm aware of is the two knights versus one pawn endgame, which in certain positions can take up to 115 moves for the knights to serve checkmate.
The 50 move-rule & 75 move-rule are to be ignored.
For the specified details (ignoring the 50-move rule, max. 6 pieces, max. 3 pieces per side), this endgame tablebase page, gives the following position, with checkmate in 262 moves:
6k1/5n2/8/8/8/5n2/1RK5/1N6 w - - 0 1
1. Kd3 N3g5 2. Kd4 Ne6+ 3. Kd5 Nc7+ 4. Kc6 Ne6 5. Kd7 Nfg5 6. Kd6 Kg7 7. Ke5 Nc5 8. Kf5 Nge4 9. Ra2 Nd6+ 10. Ke5 Nf7+ 11. Kd5 Nd3 12. Kd4 Nb4 13. Ra4 Nd8 14. Kc4 Nbc6 15. Kd5 Ne7+ 16. Kd6 Ng6 17. Ra8 Nf7+ 18. Ke6 Nf8+ 19. Kd5 Ng6 20. Ra6 Nf4+ 21. Kd4 Nd8 22. Nc3 Nde6+ 23. Ke3 Ng2+ 24. Kf2 Ngf4 25. Kf3 Kf7 26. Ra7+ Kg6 27. Ke4 Kg5 28. Ra5+ Kg4 29. Ke5 Nc5 30. Nd1 Ng6+ 31. Kd5 Nb3 32. Ra2 Nf4+ 33. Ke5 Nc5 34. Ne3+ Kf3 35. Ra3 Ng6+ 36. Kd5 Nd3 37. Kd4 Nde5 38. Ra6 Nf4 39. Nf1 Nf7 40. Rf6 Ng5 41. Ke5 Ngh3 42. Rf7 Ke2 43. Ng3+ Ke3 44. Nf5+ Kf3 45. Nd6 Ke3 46. Nc4+ Kd3 47. Nb2+ Kc2 48. Na4 Nd3+ 49. Kd4 Ng5 50. Rd7 Ne6+ 51. Kd5 Ng5 52. Rg7 Kb3 53. Nb6 Nb4+ 54. Ke5 Nd3+ 55. Kf5 Nh3 56. Rc7 Nb4 57. Rc8 Nf2 58. Nd7 Nc2 59. Nc5+ Kb4 60. Ke5 Ng4+ 61. Kf4 Nge3 62. Nd3+ Kb3 63. Nc1+ Kb4 64. Ke4 Nc4 65. Rg8 Nb6 66. Rg2 Na3 67. Rb2+ Kc5 68. Nd3+ Kc6 69. Rg2 Na4 70. Rg5 Nc3+ 71. Ke5 Na4 72. Ke6 Nb5 73. Nb4+ Kb6 74. Nd5+ Ka5 75. Rh5 Nc5+ 76. Ke5 Nd3+ 77. Ke4 Nc5+ 78. Ke3 Na6 79. Rh1 Nbc7 80. Nf4 Kb4 81. Kd4 Nb5+ 82. Ke5 Nc5 83. Nd5+ Kb3 84. Rh3+ Kc4 85. Rh4+ Kd3 86. Nf4+ Kd2 87. Rh2+ Kc3 88. Kd5 Kb4 89. Rh4 Na4 90. Nd3+ Kb3 91. Rg4 Kc2 92. Nf4 Kd2 93. Rg7 Ke3 94. Ng2+ Kd2 95. Rh7 Nb2 96. Rd7 Ke2 97. Rb7 Nd1 98. Kc5 Nbc3 99. Kd4 Kd2 100. Rd7 Ke2 101. Nh4 Nb5+ 102. Kc4 Na3+ 103. Kb3 Nb1 104. Nf5 Nf2 105. Kc2 Na3+ 106. Kc3 Nb1+ 107. Kd4 Nd2 108. Re7+ Kf3 109. Rf7 Nh3 110. Nd6+ Ke2 111. Re7+ Kf3 112. Re3+ Kg4 113. Ra3 Nf1 114. Ra4 Ng5 115. Ke5+ Kf3 116. Nf5 Nd2 117. Ra3+ Kf2 118. Ra2 Ke2 119. Kd4 Nge4 120. Ne7 Kf3 121. Nd5 Nb3+ 122. Kc4 Nc1 123. Ra3+ Kf2 124. Ra8 Ne2 125. Rg8 Ng1 126. Kd4 Nd6 127. Ke5 Nc4+ 128. Ke4 Nd6+ 129. Kf4 Ne2+ 130. Ke5 Nb5 131. Rf8+ Ke1 132. Rd8 Nbc3 133. Nb4 Kf2 134. Nc2 Nc1 135. Rc8 Nb5 136. Rb8 Nc3 137. Rd8 Ke2 138. Kd4 Nd1 139. Na1 Kd2 140. Ke4+ Ke2 141. Rc8 Nf2+ 142. Kd4 Ncd3 143. Nb3 Nf4 144. Rg8 Kf3 145. Nd2+ Ke2 146. Nc4 Kf3 147. Ne5+ Ke2 148. Re8 N2h3 149. Nc4+ Kf3 150. Ra8 Ng5 151. Ra3+ Ke2 152. Ke5 Nd3+ 153. Kf5 Nh3 154. Ra2+ Kf3 155. Ra8 Nhf2 156. Re8 Nd1 157. Nd2+ Kf2 158. Ke4 Nc1 159. Nb1 Ke2 160. Kf4+ Kd3 161. Rd8+ Ke2 162. Rd2+ Ke1 163. Rh2 Ne2+ 164. Kf3 Nd4+ 165. Ke4 Nb3 166. Ra2 Nf2+ 167. Kf3 Nd4+ 168. Ke3 Nf5+ 169. Kf4 Nd4 170. Ra4 Nc2 171. Kf3 Nd3 172. Rh4 Kd1 173. Nc3+ Kd2 174. Ne4+ Kc1 175. Ke2 Ne5 176. Nc5 Na3 177. Re4 Nec4 178. Kd3 Nb2+ 179. Kc3 Nb1+ 180. Kd4 Na3 181. Rh4 Nb5+ 182. Ke5 Kc2 183. Rh2+ Kc3 184. Kd5 Nd1 185. Rh3+ Kb2 186. Nb7 Na3 187. Nd6 Nc3+ 188. Kc5 Kc2 189. Nf5 Nab1 190. Kd4 Nb5+ 191. Kc4 N5a3+ 192. Kc5 Kb2 193. Rh2+ Kb3 194. Nd4+ Kc3 195. Ne2+ Kb2 196. Ng3+ Kc3 197. Rh3 Kd3 198. Nf5+ Kc2 199. Rg3 Kb2 200. Ne3 Nd2 201. Rg2 Kc1 202. Kd4 Nf3+ 203. Kc3 Nb1+ 204. Kb4 Nbd2 205. Rg8 Ne4 206. Rd8 Nf2 207. Kc3 Ne4+ 208. Kd3 Nc5+ 209. Kc4 Ne4 210. Rd1+ Kb2 211. Rd5 Ne1 212. Rd8 Nf3 213. Rf8 Ne1 214. Rb8+ Kc1 215. Kd4 Nd2 216. Nd5 Kd1 217. Re8 Ng2 218. Kd3 Nb1 219. Re7 Ne1+ 220. Kc4 Ng2 221. Kb3 Nd2+ 222. Kb2 Nc4+ 223. Kc3 Nd2 224. Nf6 Nf4 225. Rd7 Ne2+ 226. Kd3 Nb1 227. Ke3+ Ke1 228. Re7 Kd1 229. Rh7 Nc1 230. Ne4 Na3 231. Nf2+ Kc2 232. Rc7+ Kb2 233. Nd1+ Kb1 234. Rc8 Nc2+ 235. Kd2 Nb3+ 236. Kd3 Ne1+ 237. Kc3 Kc1 238. Rd8 Nc5 239. Nf2 Na4+ 240. Kb3 Nb2 241. Rc8+ Kd2 (241... Kb1 242. Rc3 Na4 (242... Ng2 243. Ne4 Nd3 244. Nd2+ Ka1 245. Rc4 Nc5+ 246. Rxc5 Ne3 247. Rc1#) 243. Kxa4 Nc2 244. Nd3 Nd4 245. Rc1+ Ka2 246. Kb4 Nb5 247. Kxb5 Kb3 248. Kc5 Ka3 249. Kc4 Ka2 250. Rd1 Ka3 251. Ra1#) 242. Kxb2 Ke3 243. Ng4+ Kf4 244. Ne5 Ng2 245. Ng6+ Kg5 246. Rc3 Kg4 247. Rd3 Ne1 248. Rd4+ Kh5 249. Nh4 Nd3+ 250. Rxd3 Kxh4 251. Rd5 Kg3 252. Rd4 Kf3 253. Kc2 Ke3 254. Rc4 Kf2 255. Rc3 Ke1 256. Kd3 Kf2 257. Kd2 Kg1 258. Ke3 Kg2 259. Ke2 Kh1 260. Rg3 Kh2 261. Kf2 Kh1 262. Rh3#
Longest known checkmate is in 549 moves, revealed by Lomonosov Tablebases.
You can see the game here.
Plus, you don't need to say "endgame" explicitly. Longest checkmates come from endgame tablebases already. And it's not possible to build a middlegame tablebase practically, because of the exponential space complexity. Let's say there is a middlegame position which is in 12-men tablebase:
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5-Men | ~7GB
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6-Men | ~1200GB
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7-Men | ~140000GB
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8-Men | ~10000000GB Estimated
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12-Men | Do the math :-)
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The longest known forced checkmate with disregard for the 50-move rule is a mate in 549. You can replay at http://tb7.chessok.com/probe/745/61.
[Event "Longest 7-piece checkmate in chess"]
[Site "GothicChess.info"]
[Date "????.??.??"]
[Round "?"]
[White "?"]
[Black "?"]
[Result "1-0"]
[SetUp "1"]
[FEN "1n1k4/6Q1/5KP1/8/7b/1r6/8/8 w - - 0 1"]
1.Kf5 Rb5+ 2.Kg4 Rb4+ 3.Kf3 Nd7 4.Qh8+ Ke7 5.g7 Bf6 6.g8=N+ Ke6 7.Qh3+ Kd6 8.Kg2 Rb2+ 9.Kf1 Rb1+ 10.Ke2 Rb2+ 11.Kd1 Rb1+ 12.Kc2 Rb2+ 13.Kc1 Rb5 14.Qh6 Rc5+ 15.Kb1 Rb5+ 16.Ka2 Ra5+ 17.Kb3 Rf5 18.Ka3 Rf3+ 19.Ka2 Ke6 20.Qh7 Ne5 21.Nh6 Rf2+ 22.Kb1 Rf4 23.Qg8+ Kd6 24.Ka2 Nc6 25.Ng4 Bd4 26.Qg6+ Kd5 27.Qg5+ Ke4 28.Kb1 Nb4 29.Qe7+ Kd3 30.Qh7+ Ke2 31.Qh2+ Kf3 32.Qh3+ Ke4 33.Qg2+ Kd3 34.Qg3+ Ke4 35.Qe1+ Kd3 36.Qd1+ Ke4 37.Qe2+ Kd5 38.Kc1 Kd6 39.Qh2 Nd5 40.Kd2 Bc3+ 41.Ke2 Bd4 42.Qh6+ Kd7 43.Qh7+ Kc6 44.Qg8 Kc5 45.Kd3 Nb4+ 46.Kd2 Nd5 47.Qc8+ Kd6 48.Nh6 Bc3+ 49.Ke2 Re4+ 50.Kf2 Rf4+ 51.Kg1 Bd4+ 52.Kh2 Ne3 53.Qd8+ Kc6 54.Qe8+ Kc5 55.Nf7 Rf2+ 56.Kh3 Rf3+ 57.Kh4 Rf5 58.Nh6 Rf1 59.Qa8 Kb4 60.Kh5 Kc3 61.Qc8+ Kd3 62.Kg6 Rf2 63.Qa8 Rf1 64.Qb8 Kd2 65.Qb3 Ke2 66.Qb5+ Kd2 67.Qg5 Kc2 68.Kh7 Kd3 69.Qb5+ Kc3 70.Qa5+ Kd3 71.Qg5 Rf6 72.Qb5+ Ke4 73.Qb7+ Kd3 74.Qd7 Kc3 75.Qe8 Kd2 76.Qb5 Kc2 77.Qe2+ Kc3 78.Qh5 Rf1 79.Qg6 Rf6 80.Qe4 Kc4 81.Qe8 Kd3 82.Ng8 Rf1 83.Ne7 Rh1+ 84.Kg6 Rg1+ 85.Kf7 Rf1+ 86.Ke6 Rf6+ 87.Kd7 Bc5 88.Nc6 Nd5 89.Nd8 Rd6+ 90.Kc8 Ne7+ 91.Kc7 Nd5+ 92.Kb7 Rb6+ 93.Kc8 Ne7+ 94.Kd7 Nd5 95.Qe1 Rd6+ 96.Kc8 Be3 97.Qd1+ Ke4 98.Qc2+ Kd4 99.Qa4+ Ke5 100.Qh4 Kf5 101.Qh3+ Ke4 102.Nf7 Rc6+ 103.Kd8 Kd4 104.Ke8 Rc7 105.Kf8 Bf4 106.Qe6 Ra7 107.Qe2 Be3 108.Qg4+ Kc5 109.Qc8+ Kd4 110.Qb8 Kd3 111.Kg8 Rc7 112.Qb5+ Kd4 113.Qa4+ Kc5 114.Qa6 Kd4 115.Kf8 Bd2 116.Qf1 Bc3 117.Kg7 Rc6 118.Kg8 Nf6+ 119.Kf8 Rc8+ 120.Kg7 Ne4 121.Qb5 Ke3+ 122.Kh7 Rc7 123.Qb6+ Rc5 124.Qe6 Kf4 125.Qh6+ Kf3 126.Qh3+ Ke2 127.Kg8 Rc6 128.Kf8 Bb4+ 129.Kg7 Bc3+ 130.Kg8 Rf6 131.Qh4 Kd3 132.Nd8 Kd4 133.Qg4 Ke5 134.Qe2 Kf4 135.Qc4 Kf3 136.Qb3 Kf2 137.Ne6 Ke3 138.Qa2 Rf3 139.Qc2 Be5 140.Qd1 Bc3 141.Qc1+ Kd3 142.Nf4+ Kc4 143.Ng2 Rf2 144.Nh4 Nf6+ 145.Kg7 Ne4+ 146.Kg6 Rf6+ 147.Kh5 Kd4 148.Qd1+ Ke3 149.Qb1 Bd2 150.Ng6 Ng3+ 151.Kh6 Bc3 152.Qc1+ Kd3 153.Qg1 Ne4 154.Kh7 Rd6 155.Kg8 Rd8+ 156.Kf7 Rd7+ 157.Ke6 Rd6+ 158.Ke7 Bf6+ 159.Ke8 Rd8+ 160.Kf7 Rd7+ 161.Ke6 Rd6+ 162.Kf5 Rd5+ 163.Kf4 Bg5+ 164.Kg4 Ke2 165.Qa7 Bd2 166.Qe7 Rg5+ 167.Kh3 Ke3 168.Nh4 Rg3+ 169.Kh2 Rg4 170.Qa3+ Bc3 171.Qa7+ Ke2 172.Ng2 Be5+ 173.Kg1 Nd2 174.Qf2+ Kd1 175.Kh1 Bd4 176.Qe1+ Kc2 177.Qe2 Re4 178.Qh5 Nf1 179.Qd5 Nd2 180.Qf5 Kd1 181.Nf4 Be5 182.Qh5+ Kc2 183.Ng6 Bf6 184.Kg2 Bg5 185.Qh7 Kd1 186.Qh8 Be3 187.Qh5+ Kc2 188.Nh4 Bd4 189.Nf5 Be5 190.Qe8 Rg4+ 191.Kh3 Re4 192.Qc8+ Kd1 193.Qc5 Nc4 194.Nh6 Nd2 195.Ng4 Bd4 196.Qb5 Rf4 197.Kh4 Be3 198.Qd3 Bd4 199.Kh5 Re4 200.Qa3 Rf4 201.Qd6 Re4 202.Nh6 Bf2 203.Kg6 Rc4 204.Qe5 Bb6 205.Qg3 Rc6+ 206.Kg7 Rc4 207.Nf7 Bc7 208.Qg1+ Kc2 209.Nh6 Be5+ 210.Kg8 Re4 211.Nf5 Bf4 212.Qh1 Be5 213.Qh3 Kd1 214.Kf8 Bf4 215.Qg4+ Ke1 216.Qg2 Be5 217.Qh1+ Ke2 218.Ke8 Bg3+ 219.Kd7 Bf2 220.Ne7 Rd4+ 221.Nd5 Kd3 222.Kc6 Rh4 223.Qg2 Rc4+ 224.Kb5 Be3 225.Qh1 Bc5 226.Qh7+ Ke2 227.Qh5+ Ke1 228.Kc6 Bd4+ 229.Kd6 Bf2 230.Kd7 Rd4 231.Qh1+ Ke2 232.Ke6 Rh4 233.Qg2 Rc4 234.Ke7 Nf3 235.Qh3 Re4+ 236.Kd8 Rd4 237.Qh5 Bg3 238.Qe8+ Be5 239.Qb5+ Kf2 240.Kc8 Re4 241.Kb7 Bd4 242.Qc6 Rg4 243.Ka6 Rg7 244.Nb4 Re7 245.Nd3+ Ke3 246.Qc1+ Ke4 247.Nb4 Re5 248.Qb1+ Ke3 249.Qc2 Rc5 250.Qd3+ Kf4 251.Qe2 Kg3 252.Qe7 Rh5 253.Nd3 Rh6+ 254.Ka5 Rh4 255.Qe6 Bc3+ 256.Ka6 Bd4 257.Qc6 Be3 258.Qe8 Bd4 259.Qb8+ Kg2 260.Qg8+ Kh3 261.Qg6 Rg4 262.Qc6 Kg3 263.Qc7+ Kh3 264.Nf4+ Kh4 265.Qd6 Kg3 266.Ng6+ Kf2 267.Qe6 Rg1 268.Nf4 Ra1+ 269.Kb7 Ra7+ 270.Kb8 Ra3 271.Qe2+ Kg3 272.Nd3 Rb3+ 273.Kc8 Rb6 274.Qe4 Rf6 275.Kd7 Bc3 276.Nc5 Bd4 277.Ne6 Be5 278.Ng5 Bb2 279.Qe3 Kg4 280.Ne4 Rf7+ 281.Kc6 Rf5 282.Nf2+ Kg3 283.Nd3 Be5 284.Kd7 Rf7+ 285.Ke8 Rf5 286.Qc5 Kg4 287.Nf2+ Kg5 288.Qd5 Kg6 289.Nd3 Bg7 290.Qe4 Kg5 291.Ke7 Bd4 292.Kd7 Rf6 293.Kc8 Rf8+ 294.Kb7 Rf7+ 295.Ka6 Rf6+ 296.Ka5 Bc3+ 297.Ka4 Ra6+ 298.Kb5 Ra5+ 299.Kb6 Rf5 300.Qe7+ Kg6 301.Qe8+ Kg5 302.Kb7 Bd4 303.Qe7+ Kh5 304.Qe4 Rf7+ 305.Ka6 Rf6+ 306.Kb5 Kg5 307.Nb4 Be5 308.Nd5 Rf5 309.Qe3+ Kh5 310.Ka6 Rf7 311.Qf2 Ng5 312.Qc2 Nf3 313.Qc8 Kg5 314.Qe6 Rf5 315.Kb7 Bd4 316.Qe7+ Bf6 317.Qe3+ Kh5 318.Qe4 Kg5 319.Nc7 Nd4 320.Qg2+ Kh5 321.Nd5 Nf3 322.Ne3 Rf4 323.Qh3+ Nh4 324.Qe6 Kg5 325.Qd5+ Kg6 326.Qd6 Kg5 327.Qc5+ Kg6 328.Qc7 Kg5 329.Nd5 Rf3 330.Qc1+ Kg4 331.Ne3+ Kg5 332.Nc4+ Rf4 333.Nb6 Kg4 334.Qd1+ Kg3 335.Nd5 Rf3 336.Qd2 Kg4 337.Ne3+ Kh5 338.Nc4 Bg5 339.Qd1 Bf4 340.Nb6 Bg3 341.Nd7 Kg5 342.Qd5+ Rf5 343.Qe4 Rf4 344.Qe7+ Kg4 345.Qe6+ Kf3 346.Nf6 Kg2 347.Qd5+ Kh2 348.Ne4 Nf5 349.Ng5 Nd6+ 350.Kb6 Bf2+ 351.Kc7 Bg3 352.Kd7 Nf5 353.Qd2+ Rf2 354.Qd1 Rf4 355.Qe2+ Rf2 356.Qh5+ Kg2 357.Kc6 Rc2+ 358.Kd5 Rd2+ 359.Kc4 Rf2 360.Qh3+ Kg1 361.Kb4 Rf1 362.Ka5 Rf4 363.Ka6 Ra4+ 364.Kb5 Rf4 365.Ka5 Rf2 366.Qh8 Kg2 367.Qc3 Rf4 368.Qd2+ Kf1 369.Qd5 Nd4 370.Kb6 Bf2 371.Kc7 Rh4 372.Qe5 Kg2 373.Ne4 Kf3 374.Nd2+ Kg2 375.Qd5+ Kh2 376.Kb7 Rh7+ 377.Ka6 Rh6+ 378.Ka5 Rh4 379.Qd6+ Kh3 380.Qd7+ Kg2 381.Qb7+ Kg3 382.Qg7+ Rg4 383.Nf1+ Kh3 384.Qd7 Kh4 385.Qh7+ Kg5 386.Qg7+ Kf4 387.Qc7+ Kg5 388.Nd2 Rf4 389.Qg7+ Kf5 390.Nc4 Nf3 391.Nd6+ Ke6 392.Ne8 Kf5 393.Ka6 Bd4 394.Nd6+ Ke6 395.Qg8+ Kf6 396.Qh7 Ke6 397.Ne4 Kd5 398.Qb7+ Ke6 399.Ng3 Be5 400.Qb3+ Kd6 401.Qb6+ Ke7 402.Ne4 Kf7 403.Qh6 Kg8 404.Qe6+ Kh8 405.Qe7 Rf5 406.Kb6 Bg7 407.Ng3 Rf6+ 408.Kb5 Nd4+ 409.Kc4 Rc6+ 410.Kd3 Rc3+ 411.Ke4 Rc6 412.Qe8+ Kh7 413.Kd3 Rd6 414.Qh5+ Kg8 415.Ne4 Rd8 416.Ke3 Bh6+ 417.Kf2 Rf8+ 418.Kg2 Kh7 419.Ng3 Ne6 420.Qh4 Ng7 421.Qe4+ Kg8 422.Qd5+ Kh7 423.Qd6 Rf7 424.Qd3+ Kg8 425.Qd5 Bc1 426.Kh3 Ba3 427.Kg4 Be7 428.Qc4 Kf8 429.Qc8+ Ne8 430.Nf5 Rf6 431.Qd7 Rf7 432.Qd3 Bf6 433.Qe3 Be7 434.Qh6+ Kg8 435.Qh1 Rh7 436.Qd5+ Kf8 437.Kf3 Nf6 438.Qe5 Ne8 439.Ke2 Bf6 440.Qd5 Rf7 441.Kd2 Bg5+ 442.Kc2 Rc7+ 443.Kb2 Bf6+ 444.Kb1 Rh7 445.Qf3 Ra7 446.Qb3 Rc7 447.Qb4+ Kf7 448.Qb6 Rd7 449.Qb3+ Kf8 450.Nh6 Rg7 451.Qa3+ Be7 452.Qf3+ Bf6 453.Ng4 Ke7 454.Kc2 Rg5 455.Qe3+ Kf7 456.Qb3+ Kf8 457.Qe6 Bg7 458.Ne3 Re5 459.Qb6 Bf6 460.Ng4 Rf5 461.Qe6 Rf4 462.Qc8 Bd4 463.Qd7 Bg7 464.Ne3 Rf2+ 465.Kb1 Rf7 466.Qc8 Rc7 467.Qa6 Rd7 468.Qa3+ Kf7 469.Qb3+ Kf8 470.Qb4+ Nd6 471.Qf4+ Kg8 472.Nd5 Nf7 473.Qc4 Ra7 474.Nf4 Rb7+ 475.Kc2 Rb2+ 476.Kd1 Rb6 477.Qc8+ Kh7 478.Qc7 Rd6+ 479.Ke2 Kg8 480.Qe7 Bf8 481.Qe8 Ng5 482.Qc8 Rf6 483.Qg4 Bh6 484.Nh5 Re6+ 485.Kf1 Re5 486.Qc8+ Bf8 487.Qc2 Re6 488.Qa2 Kh7 489.Qg2 Kh6 490.Qh1 Rb6 491.Nf4+ Kg7 492.Qh5 Nh7 493.Qg4+ Kf7 494.Qd7+ Be7 495.Qd5+ Kf8 496.Ne6+ Ke8 497.Qh5+ Kd7 498.Ng7 Nf6 499.Qf5+ Kc7 500.Qe5+ Kd7 501.Qd4+ Kc7 502.Qe3 Nd5 503.Qe5+ Kc6 504.Nf5 Rb7 505.Qe4 Kc5 506.Qc2+ Kb6 507.Qb3+ Kc6 508.Nd4+ Kc5 509.Qxb7 Kxd4 510.Kf2 Ke4 511.Qb1+ Kd4 512.Qg6 Bf6 513.Kf3 Be7 514.Qg1+ Kc4 515.Qg4+ Kc5 516.Ke2 Nf6 517.Qf5+ Kd4 518.Qf4+ Kd5 519.Kf3 Ke6 520.Qc4+ Kf5 521.Qa6 Nd7 522.Qd3+ Ke6 523.Qe3+ Kd6 524.Kf4 Nc5 525.Kf5 Bf8 526.Qf3 Nd7 527.Qc3 Ke7 528.Qd4 Ke8 529.Ke6 Nc5+ 530.Kd5 Nd7 531.Kc6 Nb8+ 532.Kb6 Nd7+ 533.Kc7 Nc5 534.Qc4 Be7 535.Qg8+ Bf8 536.Kc6 Na6 537.Qe6+ Be7 538.Qc8+ Kf7 539.Qxa6 Ke6 540.Qd3 Bg5 541.Qe4+ Kf6 542.Kd6 Bd2 543.Qe5+ Kg6 544.Ke6 Bg5 545.Qf5+ Kh5 546.Qf3+ Kh6 547.Qf7 Bh4 548.Kf5 Bg5 549.Qg6#
Check this link from the Encyclopedia of Integer Sequences - OEIS: https://oeis.org/A260954 In short, the results are as follows:
Pieces / Longest moves / FEN:
3 pieces, 28 moves: 8/8/8/1k6/8/8/K5P1/8 w - - 0 1
4 pieces, 43 moves: 8/5k2/2PK4/5r2/8/8/8/8 w - - 0 1
5 pieces, 127 moves: 8/8/8/8/1p2P3/4P3/1k6/3K4 w - - 0 1
6 pieces, 262 moves: 6k1/5n2/8/8/8/5n2/1RK5/1N6 w - - 0 1
7 pieces, 549 moves: 1n1k4/6Q1/5KP1/8/7b/1r6/8/8 w - - 0 1
Results based on research on the Nalimov and Lomonosov Endgame Tablebases. The results for 6 and 7 pieces ignore the 50-move rule.
At the moment (October 2022) the longest forced mate is this one in 595 moves (disclaimer: it's my own video).
