To me that is an amount of time where it critical wether or not I want to start another game before going to bed. With Binding of Isaac I can just quit the game after 20 minutes, or die 10 times after 2, I don't have to worry, and just keep enjoying the game. When I have to decide beforehand, knowing it would keep me up another 30-50 minutes if I was successful, I'd just go to bed.