20 mins is a bit harsh, I'd say drop it to 10-15 mins after the first withdraw. That sounds like a reasonable punishment for withdrawing, since those with actual legitimate reasons to withdraw can probably take care of whatever it is in 15 mins and requeue without issue. And as for the people just looking for quick tomes... well, they're already irritable enough about people watching three minute cutscenes. I'd think 15 mins would be enough of a deterrent to teach them a little patience.