Depends?
If you are pre-booking a few days in advance, then 24 hours is reasonable.
If you are trying to find a lunchtime treat then the below is a good approach.
An hour is sufficient, and then it's fine to move on to Plan B. If the original provider gets back to you after you book an appointment with a back-up, you politely say you were in need of a short notice session and reached out to someone else when you weren't available, but will be happy to try again in the future.
Originally Posted by TittiesNBeer
If you need someone RIGHT NOW then you should reach out via text/call to the ladies that have already seen or screened you. Text one you'd like to see and one who is reliable,
when one confirms IMMEDIATELY give the other one notice you are no longer looking but will try seeing her again in the future. The bold is important don't forget to do that or you could be labeled a time waster.