For various reasons that we're SLOWLY working on, my current girlfriend usually finds the process painful; sometimes only mildly discomforting; rarely more than slightly-pleasuring... I've fallen into the groove of having my orgasm within 5-10 minutes so that I can switch to doing things that she actually finds pleasurable.

With previous girlfriends, anywhere from 10 to 30 minutes usually. I couldn't fathom going longer--unless lots of breaks and changes in position were happening. I haven't been with a woman that wanted me to go longer, but I'm definitely curious as to how that works and how they make it work.