Take my opinion with a grain of inexperienced salt (as I have only ever got FS from a ML when she offered it at a cost).
I don't know if any unspoken rules but the way I see it there is 2 sides to this situation that both lead to the TLDR answer of "it doesn't matter"
1. It is agreed to/offered as a paid extra
In this case the ML seems to be more interested in the money than the sex, to which I respond with 'fuck her for as long as you want/can'. You paid the money you should get the value for it.
2. Sessions leads to FS
If the session leads to sex it normally means the ML is happy for it and/or wants it. From the sounds of it most of the time this happens there isn't even an extra cost. This case is more an act of passion so I say just make the experience for her and you memorable, enjoyable and erotic, whether that be pumping for 5-10min in one position or 30min over multiple positions.