For some reason (pretty sure it is because I used to beat my meat a lot), I almost never finish while having sex or BJ or HJ (from the girl) so the session lasts as long as my body can manage so I've gone for 40 mins some times (never with a WG, she'll probably give me the death stare if i took too long).
That being said, there have been a few times (<10) where I have managed to

while smashing and I felt amazing. I've even done it w/o a condom with my exgf and we went at it for 4 days while staying indoors, but I didn't even

even once (because I was hell bent on not rubbing one out myself).
Going to WGs I always finish it off myself after all the service. However, there was just one HG where I could

normally and another where I blew from a BBBJ; both times it was absolutely fantastic.
I suppose it has it's pros and cons.