So many variables.
 

Have they been edging? Did they cum 30 minutes ago? Is it time for them to go or did they just get there and want to sample a few cocksuckers before nutting? How good is your technique? Is he distracted or does he have ED? Possibilities are endless.

+/- 10 minutes.
 

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. :D

s4c -
I think you're right-on with the variables thing. I've actually never even thought about the subject. I guess I just figure he'll cum when I've done my job sucksessfully. Of course I have had the ones that take forever - and - sadly even those that never give up the creamy nectar. But, really those are the exceptions to the rule. The thought of a cocksucker checking his wristwatch mid-BJ is really kinda a boner-kill. Even worse that the cocksucker in a BJ-vid who is always checking the monitor to see how he/she looks.