About 30 seconds. I'm not exaggerating in the least. One was orally at a place called Club Orlando. It was an older guy that invited to my room. About 10 seconds into it, he said to me "ease up a little, you'll make me cum". About another 10 seconds went by and he said "oh, fuck it, I can't hold it" and erupted like a volcano.
After it was over, he said to me "somebody taught you well".
Oddly enough, I have sucked a cock for 2 hours without the guy even getting close to cumming, which is frustrating.
The other was when I rode the cock of a guy I met on Craigslist. That was another adventure that lasted about 30 seconds. He was embarrassed and swore it would never happen again.
I gave him a second chance about a week later and the same thing happened. He said to me "this normally don't happen, you're a fuck machine". That's probably true. When I ride a cock, I do it hard and aggressively. That's what gets me off.
I saw no point in trying for a third time after that.