Personally I'd give 48 hours.
Not only would I like to hear from the gentlemen but from the providers as well. I like to schedule my appts from 24-48 hrs in advance. Even doing so I would have to say there's a 50/50 chance of the appointment starting on time if at all. Why is this, and are there better options/ways to schedule.
It depends:
1. have you made contact with her before
2. have you been screened/cleared by her
3. what are her requirements
(we have lives too - ie jobs, family/kids, etc or tour so availability may be limited)
So if there is a lady you are interested in - contact her, get cleared, and then ASK HER how much notice she needs. That should work the best.
Misty
Yes, I am talking about when one has previously met thier provider and both parties have agreed to appointment procedures.
When I contact a provider fot the 1st I will follow the procedures on their website/AD, but even so I expect a few hiccups. I never attempt to make contact with providers who want you to contact them only when you're ready as that's normally not set up for a relaxing enjoyable time.
Just wondering experiences and ideas
each lady is different.
Many have limited schedules due to jobs, kids, family, lives.
But remember "things happen". You may plan 24 hrs ahead but then have family emergency, get sick, have to work, etc.
Nothing is for sure here OR in life....except death and taxes 
Personally I'd give 48 hours.
I would say it's up to the provider and also the time of year. Some ladies like those who plan way in advance and other like 24-48 hrs in advance.
Also if she lives in a major city and has high reviews, she might be busier on certain months for example summer. When a lot of people are on vacations.
So again it's really up to the lady and how she wants to set up dates.
When I want to see my ATF I often contact her 4-5 days in advance because I am early morning guy and I know that I need to let her know well in advance. As long as I do that she has never failed me. As such we have a great rapport. With somebody I haven't seen I still contact 2-3 days in advance but I am more open to her scheduling needs. Part of that for me is that if things don't work out it's not on me. Once I make an appointment I don't cancel and I truly don't know what being late is. If we don't meet it's her failure and I move on. I may miss meeting some great ladies because of this, but bottom line is I respect their time they should respect mine as well. That said, if I get a call well in advance and there is good communication I am open to re-scheduling one time. What I abhor is being on time and sitting in a parking lot somewhere waiting for a call. There is plenty of pay 4 play pussy out there why should I wait for hers.