I'd say it doesn't take that long. It can happen fairly fast depending on the sexual activity that's causing it. Like if a girl is teasing him good and keeps it up, (this doesn't have to include rubbing!) after he has had a boner for, let's say 15 minutes, if she suddenly stops and then walks off, he'll be left with a huge urge to get off. But if he's where he can't do anything to relieve himself, he'll start getting soft andbe left with this feeling like a numb leg that was asleep and now waking - a weird prickly feeling and precum that's made his pants wet. When you suddenly leave a guy like that after having teased him hard for awhile you'll have blue balled him. Maybe some guys will have needed some teasing by rubbing first, and depending on how good the girl was as teasing, then when the girl suddenly stops and leaves him in that situation, she'll have left him blue balled.