A general rule of threes for survival is that to stay alive you must; Breathe air within three minutes; Find warmth/shelter within three hours; Drink water within three days; Eat food within three weeks!
Obviously, there are always exceptions and people having different abilities or maybe animals 😀
It depends very much on body temperature but 3-4 minutes is the about the most. Some extreme free-divers can stay down for longer but they breathe oxygen for some time beforehand. I remember one woman who survived an hour trapped under water in an apline stream because the cold water cooled down her brain – but I think she took about a year to recover. So the answer is, not long!