For most openings that I play (e4 and d4 openings) I have observed that they take exactly 4 moves (e.g. Giuoco Piano Game without variation).
C50: Giuoco Pianissimo, Italian four knights variation takes 5 moves. The Van't Kruijs Opening (1. e3) takes less than 1.
Which opening requires the longest amount of chess moves?
As I see I have been unclear, let me clarify. When I say two distinct openings, all I mean is a different name. For example, Ruy Loypez, XXX Variation is different from Ruy Lopez Ppening is different from Ruy Lopez, YYY Variation.