I never leave it until the last minute.

Like Yahillwe, I have done whatever I need to do (note - I do not shave my legs in the on board bathroom!) and I am back in my seat long before the 40 minutes to landing announcement.

Why does everybody wait until the seat belt signs are about to be switched on to go? Poor bodily function management!

In answer to the actual question. Everybody should spend as much time as they need to doing the things that they have to do on board. Need does not include everything that one might do in a bathroom at home - only things that have to be done.

I have sympathy with folks who have to hog a bathroom - I once used one for almost an entire TWA flight when I came down with a bout of Montezuma's revenge, but the people who make me extremely annoyed are the drunks who have a skinful before they even board, and then throw up all over the bathroom, putting it out of action for the entire flight. (Thank you Mr 5E on a recent flight. You really can't drink with the big boys and you made things very unpleasant for your cabin mates.)