With 5 years of being together, there's only 1 time I can ever remember us giving up when it comes to him giving me oral. Oral for him never works because he gets too excited to do it. Lol. But I believe we ended up giving up only once and it was because I was so exhausted that I couldn't get there.
Sometimes I'll get there in 5-10 minutes, only every once in a while though. Then sometimes it takes a good 30 which is pretty common for me. Even if I tell him he can quit, if it's taking too long, he's determined! Lol. He won't get up until I'm there. Hardly ever takes longer than 45 min. though unless he decides to go for 2!