How many innings does a pitcher have to pitch to get a save in the MLB?

A single out is enough to get a save. To get a save a pitcher has to record the game ending out, and has to have entered with a lead of 3 runs or fewer, or with the tying run at the plate or on deck. If a pitcher throws the game's final 3 innings he can get the save regardless of how big a lead his team had when he entered.