I have had some SPs conveniently steer me to the door after 2 and with some time still left (maybe 10-15 min), but the key is to manage your expectations with the SP before you meet. If you can reasonably go for 3 or 4 ask her if she is cool with that.
P.S. If any SP can coax 3 or 4 it is VL (I can't speak from experience for Sara, OB, Lisa, Pandora, Saffire and the others but they can probably work some magic also).
about a year ago i did a all niter with an sp from kamloops, within that time only 3 shots were had. I would have liked to do more since she had an awesome body but it wasn't encouraged.I need abit of time to get the little fella back to attention but 2 shots within a hour works for me.