Ladies and more experienced gentlemen!

If there is no mention of being unavailable or on vacation I would say 24-48hr waiting time for a response.
If after that time you still don't hear back perhaps follow up with one more email.Sometimes emails go into spam,get lost,etc.
But if you still get no response after the second email I would move on.In your email did you ask about a specific day and time and provide screening info in the initial email?Is your requested date soon or is a few weeks-month away?
Some might not respond as fast if the date weeks or a month away.Although I would think it would be common courtesy to acknowledge receiving an appt request if it was non explicit and provided the necessary info.

I usually respond within couple hours, I'm always on my phone checking emails.

...24 hours, I'll move on to someone else.  If she eventually gets back to me, I'll keep her in mind for a future date but I never send a follow-up email if I haven't heard from her.  To me, that's like begging and as that old philosopher GaGambler says: "I will beg for pussy, I will pay for pussy, but I will never beg to pay for pussy."  

Approximately 12 hours on a weekday and 24 hours for a weekend, depending. If it is a weekend, I might wait until Monday to hear back from her since there are quite a few providers who don't work weekends.  

I always strive to reply within 24 hours, but I do not use a booking service, and have a IRL job as well, so sometimes it's slightly longer, but I do state that on my contact page. If you send the email while I am getting ready for an overnight, and I have to work the next day, it might be just over the 24 hour mark when I reply. I do not touch my phone while on another date, unless my date asks me to look something up. If I'm going out of town on a date, I leave an auto-reply on my email, with a clear expectation as to when I will be able to reply.  

And THANK YOU for your always useful comments!

Respond my emails quick, also I have an automatic response because many times are just people that doesnt do anything with their lifes and send mass emails to all ladies with no appropriate introduction, or the common "are you available now?".
I disregard those.... at least show me that an adult wrote and sent the email.

In my opinion, for appointment requests more than a day or two in the future, it is reasonable to expect some type of response within 24 hours excluding the weekend. i.e. If I sent a request on Friday, I wouldn't expect a response until Monday.  

How well he takes direction. It's all spelled out on how to connect. If that's seemingly too difficult to do on his part, then it let's me know he and I will probably not click well so I'd rather he spend his hard earned money on another lady here. I'll let him know that right away. You usually know by first contact if a go or no go fly zone lol

in the future, then there is some time to wait, say 24-48 hours. If the date is within a day or two, then the response needs to be more timely. Those who plan ahead have more lattitude.