It isn't possible to force checkmate with King vs. King, Knight & Knight. A checkmate position, however, is still possible to achieve, so does that mean the 50 move rule applies, or does insufficient material mean insufficient material to force checkmate?
This is a complicated situation, and it is made even more complicated by the fact that USCF and FIDE rules differ slightly.
First and most importantly, the 50 move rule always applies. If 50 moves are made without a capture or a pawn move, then the game is simply declared drawn.
The rules about insufficient material only are important when one side runs out of time. Basically, the rule says that if a player runs out of time, and the opponent does not have enough material to mate, then the game is declared drawn instead of the side with time remaining getting the win. For example, if black has a lone king and white flags while trying to mate with a king and queen, the game is a draw. If there is insufficient material to deliver mate, an OTB game can also be declared drawn even with time on the clock if a tournament director is summoned. This will only apply for cases like K vs K or KN vs K. Usually it is not an issue because the opponent will simply agree to a draw.
So now to the specific case of KNN vs K:
Under FIDE rules for standard tournament time controls, if the side with just a king runs out of time, then the side with KNN wins because a mating position can be constructed. If the side with just the king avoids one move mates, however, then mate cannot be forced.
Under USCF rules, if the side with just a king runs out of time, then the game is declared drawn. The USCF focuses on the fact that checkmate cannot be forced.
Many online chess servers will automatically stop a game when there is not enough material left on the board to mate. In the specific case of KNN vs K most servers do not automatically give a draw (following the FIDE rules). One interesting case is KN vs KB when Playchess and ICC will stop the game and award a draw (not folowing FIDE rules) while Chesscube follows the FIDE rules and continues play.
Now for practical play, if you are defending with just the king, it is very easy to avoid mate for 50 moves, even if you have almost no time remaining. Make sure that you don't move to a square where mate can be delivered next move and you will never be forced into mate.
I only know FIDE rules.
K+N+N v K is a position in which checkmate is still possible, therefore it is treated exactly like any other position in which checkmate is still possible (like, say, the opening position).
In particular, this means that:
This position is not declared a draw, like dead positions are (K+B v K, for instance, is an immediate draw not requiring any claims)
If the side with only a King runs out of time, he loses
But there are a few other rules to mention besides the 50 move rule:
If it's rapid or standard chess, there is no increment, and play is in the last time period (in which there's therefore only a limited amount of time for the rest of the game), if the defending side has less than 2 minutes on the clock, he can claim a draw under Rule 10.2 claiming that the position can't be won any more in normal fashion. Rule 10.2 is always complex but in this case it's obvious that the draw claim will hold.
There is a rule against "bringing the game of chess into disrepute" and I feel that an arbiter should give a warning based on that rule to any player that knows better continuing this in a slow game.
But in Blitz, you are out of luck and need to press your clock faster than he does.
K+N+N vs. K is a draw or stalemate. The knights cannot force checkmate though helpmate is possible.
Any minimally courteous opponent will accept your offer of a draw in this situation.
Situations where mate is available but obviously cannot be forced are uncommon. E.g. KBvKB where bishops are opposite colours.
Under USCF rules, the game would most likely be declared a draw if time ran out for either side.