It varies with me Dee, sometimes can be really quite quick, sometimes much longer, no idea what causes the differences, altho I think the more relaxed and into it I am, the better, and the quicker.
If it's me going solo it can take longer, if I'm at the mercy of a guy, can be ridiculously quick.
I'm on the other side of the story tho I think now, in that I wish there were times when I wouldn't do it, sometimes I would just like a quick play etc but it's not possible, have to find the towels, clear up afterwards etc so whilst it can be great, sometimes it just does my head in....
The toy that Easy recommended is supposed to be great for it tho and it's relatively inexpensive to try out.
Good luck with it x