how far in advance should one make an appointment?

If you are looking for short notice appointments, you'll probably have better luck if you contact a provider in advance and explain you will be looking for something on short notice and/or if you already have seen the provider a few times.

I have some gentleman that book well in advance with me (right now I have some booking towards the end of May!)  but I also try and keep some time available current bookings.  It really depends on the lady, and also her schedule.  If you have a specific day and time that works best for you, ask so your time can be reserved.  

... unless I'm booking with an agency but I don't even try anymore with an Indy.

thanks for all of the input.