First, it depends on what the nature of the message was. If you are not scheduling a date - that means a specific day and time - then it is completely up to the lady whether or not she replies. Not getting a reply just means she has better things to do at that time.  
   
 If it was to make a date, then it depends on when the date is. If it is within the next 72 hours, then you should get a reply within 24 hours max (obviously if you are trying to meet within 48 hours you need a reply sooner, and maybe you should try calling) - if you do not hear back by then, make another date with someone else.  
   
 If the date is beyond three days, then waiting two days for a reply is fine. But you still should get some sort of reply within 48 hours. Keep in mind a few things. Some providers have regular schedules and know what they will be doing in a week, while others have no idea what they will be doing tomorrow. So while the delay in reply may be for selfish reasons, such as wanting to keep the schedule open for longer dates as long as she can, stringing you along, it is more likely that she is not working, or does not know yet whether she will be working. Hopefully she will reply and say it is okay, and needs to be confirmed a couple of days before the date (note to ladies - this is always a good way to reply as it gives you a soft commitment that can later be canceled without hard feelings and that is much better than just about any other option aside from a straight yes or no). Other ladies may not look at their email very often, just when they know they will be working.  
   
 So here is how I handle it.  
   
 Send introductory email asking if she can meet at a specific time, usually at least 4 or 5 days out if I have never seen her before. I also ask how she screens, suggesting a few options, and saying I will send screening info if we can meet.  
   
 If I do not get a reply after a couple of days, I send a follow up asking if she got my first email, and saying that I just wanted to check once more before making other plans. An hour or two later, assuming I have not heard back from the first lady, I start the process again with a second lady.  
   
 I never try to schedule something on short notice with a lady I have not seen before, but I know others who do that all the time. Too haphazard for me.  
   
 If I am really interested in a lady, I may reach out to her again a week later asking if she is around and working, noting that I tried to schedule something "a while back" without any reply and was hoping we still might be able to meet. There are times when the third or fourth email suddenly got a response and we met. Still not a good sign, since it suggests she is not very reliable, but I was motivated.  
   
 OTOH, I have had women reply that they are around, propose a date and time then disappear again. In fact there is a local lady who was actively advertising, and we exchanged emails on three occasions, each time agreeing to meet on a specific date, and each time she disappeared, failing to reply to my email the day before our planned meeting when I went to reconfirm. Three strikes and she is out, of course.  
   
 After a while you begin to get a feel as to what kind of provider you are dealing with, and what type you like to deal with. If you are a last minute guy, then you will just want someone who is around and ready to go on short notice. If you are prefer to set your schedule and have more notice, then you may want someone who schedules out a few days or a week, and then meets the commitment. I tend to like responsive and reliable, but there are times when young and flaky works. First impressions can be wrong. A lady I have been seeing regularly for over two years is very professional, reliable and responsive, except for the time she went NCNS on what was probably our second or third date. I was ready to move on until she replied to my email two days after the date apologizing for disappearing, and her story was strange enough that I chose to accept it, and we moved on from there (very happily for me). The point of that story is that even the best, most reliable provider may have something comes up that takes precedence over you and her work, so be patient and understanding to a degree and it can pay off. That does not mean sitting around on your hands and waiting until she graces you with a reply, though - go ahead and see someone else, while keeping the communication open. That has always worked for me.  
   
 zig

Until you request an exact appointment, it's just chatter.  Once you have propsed an appointment she should get back to you with a Yes or No...  quickly.  I'd say you should know the day before.  If you don't hear back you can send a reminder (Did you get...) or just move on.    
   
 I ussualy start my conversation with a subject of 'advance screening' and subsequently to book a date.  My subject would be a date & time