There's no hard & fast rule (if you'll pardon the puns ).
If you think you can go twice in 30 minutes (and you really care about such things) then best to mention it when booking.
There should be no question of having to vacate the premises once you've set free the hostages - you have paid for time & the fact that you shot your bolt is purely coincidental - just something that sometimes happens between consenting adults blah blah blah.
The thing about it is if you are well able to cum a second time then grand but if you really want to come a second time but your nads are as dry as a bone after the first time then don't expect the lady to give herself RSI trying to make the impossible happen for you. That'll piss her off.