How long you wait will depend on how far out you are scheduling. If it is in three days, then you should follow up in 24 hours. If it is a week or more, then wait 2-4 days.
Beyond three days, there is a fair chance the lady lost your email beneath a pile of other emails, so a reminder is in order. Be polite and considerate, remembering that email will tend to exaggerate the negative, and you do not want to go into a date with the provider thinking you are an angry little bitch. So keep it simple - "Hi, have not heard back and just want to make sure you got my last email and we are on for..."
However... there are organized providers, there are compulsive providers, and there are chaotic providers. How and when she replies to you provides important clues about how she will behave before and during your date. Are you okay if she runs an hour late? Or will sitting around waiting make you crazy? If you are very organized, maybe a bit on the compulsive side, then you may want to look for a lady who responds promptly and clearly, or at least acknowledges and apologizes when she made you wait for a reply.
On the other hand, if you tend to run late, are pretty relaxed, and would like to take the chance that a lady who seems to share those traits would also be more likely to lose track of your hour as it turns into 2, then be patient and go with it. My first favorite was like that, terrible about time and replying, but also terrible about keeping track of when our time started and ended (toward the end of our visits, we had a 90 minute date that had run over 2 1/2 hours and I was telling her we were way over time and I should go, and she was arguing with me about that, telling me I was wrong, it was fine, don't be in such a rush). Over time she got more chaotic, and I lost patience with the whole thing, so now I expect a prompt reply or at least an acknowledgement when a reply was slow. If I do not get that, I am a lot more likely to look elsewhere.
Lastly, as said above, make sure you give the lady every reason to respond. If you have a question, ask it clearly, simply, politely. Explain briefly why you would like an answer, and when you think you might get together based on her reply. I usually also say that I will not waste any more of her time until I am ready to schedule, and I do not bother her again. Even better, if you are ready, just propose a day and time, provide a little info about yourself, whatever screening info she requires, and ask for confirmation of the day and time. Also ask if she needs anything else from you. Don't ramble, but make sure you give her all the details she will need. That way she knows you are serious.
And I pretty much always use email. Of the ladies I have seen, there are only a very few that I contacted any other way, and that was because they specified it. One by phone, and four (three of which were in Europe) by text. There are a few more that I spoke to after the initial email, for one reason or another, usually at their request. Not talking about the two-call, just the scheduling process.
I use P411, so I most often email the lady first, get a tentative confirmation, and then send an appointment request through P411. In the last two weeks, I have done this with two touring ladies. In both cases, there was a lag in replying after I sent the P411 request. The first one I emailed to check after three days, and she replied saying we were on, she thought she had replied. The current one said we were on in email, send a request (we have actually emailed on and off for a few months, but our schedules never worked). It has been three days now, and the date is still over a week out. I am going to wait one more day and then will email her to confirm she got the request, and all is good. If I do not get a reply to that in a day or two (which I consider very unlikely based on our previous communication), I will move on. I always have a list of new ladies, and my regulars, that I can work through if something is not working out.
hope that helps,
zig