Her cumming at all is the least of your worries, you are paying her to get YOU off remember that! I usually like to get it on right away and get the first shot over and done with that gives you plenty of time to chill out have some fun and you can blast off again later. Its never ever how you imagine it will be but it's an exciting adventure never the less Good Luck!
The "chill out and have some fun" can be a highlight with the right SP! I couldn't find the post by Cameron Diablo (maybe it was lost when PERB lost some data), but she was pretty clear the constant hammering that some guys attempt (to maximize SOG's/hour I guess?) is enjoyed by almost no woman. Now, I'm sure there are SPs who enjoy or don't mind that, even over 2 hours, but odds are, they don't. So after your first SOG, hopefully you're with someone you enjoy chatting with

That's a good time (if you haven't already) to talk about what turns her on, learn a thing or two, and enjoy.
I booked 2 hours with someone once (in another city) and she WAS one of those "never stop" machines! Once one SOG was done, she was hard at work trying to get me hard again. After the 3rd SOG in one hour, I was the one calling it quits! There just wasn't much connection and I just wasn't feeling like I wanted to stay there for the 2nd hour (even tho I was confident she was good to go the whole time). I was older than you are now, but the bottom line is: be careful what you wish for! After that experience (which wasn't really bad, mind you: rocking body, total pornstar attitude), I realized: 2SOG is nice, 1SOG can still be fantastic,but most of all, a personal connection is a MUST. Without that connection, at least for me, the experience can leave some level of bad taste in the mouth, regardless of what she looks like, duration, # SOG, etc etc.