For me it clicked the first time, after just a few minutes. The combination of physical sensation and the psychological of giving my hole up to a friend of mine, and seeing the pleasure on his face and hearing his moaning - that flipped my switch in totality.
We fucked for a whole summer after that - he rarely asked for blowjobs once he knew that not only was my ass available, but that I freaking loved to get fucked and made our fucking all about getting him off and making him cum.
Couple other dudes I was regularly sucking off on the DL in the same time period also quickly promoted themselves once they found out my mouth wasn't the only thing I'd give up. So I got a lot of practice taking different sized cocks, learning how to use my hole to maximize their pleasure, learning what things made each guy tick and what got them off. It was a great learning experience.
And damn if I don't miss those days - some days I'd get fucked 4 or 5 times (there were 6 friends that I was hooking up with regularly). It was awesome.