Gen I: 1-7 turns (1-3 Stadium series).
Gen II: Same as Gen II.
Gen III: 1-6 turns.
Gen IV: 1-4 turns (In the Japanese version of Diamond and Pearl, the duration is 2-5 turns.)
Gen V: 1-3 turns. (In addition, a Pokémon's sleep counter is reset to its original amount when switched out, even if self-induced.)
Gen VI: Same as Gen V but the sleep counter no longer resets to its original amount when switched out.
If a Pokémon has the Early Bird Ability and they are put to sleep, the maximum number of turns they potentially could be asleep for is 3 in Gen III, the minimum number of turns a Pokémon could be asleep for is 0 in Gen III onward.
>Early Bird causes quick awakening from sleep. In the turn the Pokémon is put to sleep, the number of turns it will remain asleep is preset. If it is 6 turns, it will be halved to 3. If 3, it will be rounded to 1. If only 1 turn is preset, it is rounded to 0, causing the Pokémon to instantly wake up at the end of the same turn.