Nicotine pouches can last up to an hour, but it depends on how much you chew and the particular brand. You can leave a pouch in your mouth for as long as you want, but after a certain period of time you’ll find the flavor going away, at which point you’ll want to put in a fresh one.
My own experience is that nicotine pouches can last as little as 15 minutes, but if you’re not constantly chewing, any brand should definitely last much longer than that. By about 30 minutes you should notice the flavor starting to subside.
There have been reports of people taking nicotine pouches for 2 hours or more, but this definitely seems quite excessive and I’d expect the bag would be completely dissolved by then.