How about this below as a guide for everyone. Of course this depends on your schedule ( client) say you are alone for 3 hours then 30 mins or less and move on if you need that quickie. I going to assume you seen her before. If not the time may have to change. Remember her personal schedule can happen out of no where.
Tier 1 - Agency 1-2 hours max, also same for traveling SP and places an Ad here, today or yesterday I give it the same time or 5 hours.
Tier 2 - Traveling SP well known, gives her schedule in 2-4 weeks advanced, one week / 7 days max waiting.
Tier 3 - Local SP with website if not on Vacation, I doubt you can get in within 1-3 days booking so I say max 24-36 hours. If popular if not 12- 24 hours time limit.
Tier 4 - Traveling SP but find out she is in your area 12- 24 hours if booking 2-3 days ahead, but same day or tomorrow booking 5 hours is plenty.
Tier 5 - Local SP with website if not on Vacation, I doubt you can get in within 1-3 days booking so I say max week. If booking for next month.
Tier 6 if you are traveling you should have booked in advance but couldn't because of scheduling use same guide line as above. No different treatment than the other fellow.
Any one want to change / edit this?