It's been my experience that most guys are around the ten minute mark. I've had a few closer to twenty and a couple barely make it to five. If they have a really nice cock and I can get comfortable, I don't mind if they make me work for it. On the other hand, it doesn't bother me if they cum quickly either. I'm doing because I love it and I enjoy hearing them grunt and moan before they pump their jizz into my waiting mouth. I get off on relieving them of their load.