that depends upon what information I have been supplied with for screening but once the screening is satisfactory then it is just a matter of preference. I ask for as much notice as possible.

are you giving me notice or not? lol

I prefer to make dates 4-7 days in advance,  as long as I don't need to go through a lengthy verification process or I have seen the provider previously. Most ladies will post on their websites if they are willing to be seen on short notice, same day or next day.
It has been my experience over the years that the more desperate I am to see a provider, say they had to cancel the day of the date for a good reason, the probability of actually seeing any other provider that day is inversely proportional to my efforts i.e., the hornier I get and more frustrated, the odds increase I wont be successful. out of 10 times maybe once it happened.
I am sure it is the same for providers, you need to reset your brain to get ready to see a client, then get pretty and make arrangements for in all or outcall. It takes time.
Some providers are always ready, and see someone on the fly, it's just I don't know any of them, lol

While last minute is usually a no go for me, as long as screening can be accomplished quickly (references get back right away), and I am in town, 2-3 hours notice will usually suffice for me for incall, 3-4 hours for outcall (depending on the location.)

If they write this, have never met me and didn't fill out my page, I hit delete! Or I'd send some flip comment. (i.e. do you want me to shave my legs and brush my teeth first) .

What I do if they send a nice email to begin with. For starters, I do post that "It never hurts to ask". Sometimes, I'm at my place and/or have nothing really going on. I'm pretty quick to get ready. Sometimes, I'm in the middle of something and cannot get away.  

If I have not met them and I see they have references from people I know. I can usually get the information I need to move pretty quickly and set something up right away. If I have met them before as long as I can see the email in time. Sometimes emails do not come through quickly. I can usually schedule same day with a gentleman.  

All depends on my schedule for the week

References first of course.   Then, I have learned to simply reply: What date and time would you like?  If they request a specific date/time then I can reply: Yes, that works OR give them an alterative like the next day at the same time.