My jaw starts to hurt a little bit, but mostly my tongue starts to give out on me after a while. I keep pushing through it, but I try to do something different for a while until I recover. Doesn't always work though.
My jaw does get sore with my partner. That's only because his thing is big, I don't even have to do it for too long before it starts getting sore. But I'll just suck it up anyway as long as I please him.
That is actually one of the reasons I rarely give oral to completion to my current partner. I love giving oral but he is hard to get off that way and my jaw ends up aching. I really wish I could do it more often but it seriously hard work to get him off orally.