How long to wait before trying to book another escort? If you email on p411 or directly, and receive no response for two days, is that long enough?

I wait 24 hrs.  In this day and age when everyone is connected 100% of the times not getting a response in 24 hrs only means that they don't want to communicate  - maybe taking a break, unorganized (get too many emails), your email didn't meet their requirements..  what have you !!!  Doesn't matter.

but I think that is being fairly generous.

 
24 hours should be the limit because any gal who isn't checking email or texts within that period and at least acknowedging your query  isn't someone to try to do business with in the first place.   (Though I admit there are exceptions.)

 
Other time limit that should be thought about is how long to wait once you're at the designated location just before the date and waiting to get the room number.    Some give only 15 minutes but others up to 30 minutes.

 
Also, if she leaves the room right after you hand over the envelope to go to her car and get the condoms, might as well just get dressed and leave.   She ain't a coming back.

For me, it’s two days. After that, I move on. On a few occasions, I had providers contact me later. Every time, I learned it was a sign of what was to come.

A 24-hour response time is reasonable during the traditional workweek, sure. But weekends are different. These tragically wonderful and wonderfully tragic devices have conditioned us to expect instant communication, but not everyone is glued to their phone 24/7. And many escorts, who feel like they are chronically online to prove they're real, really value taking time offline. Some of us are juggling other careers, businesses, school, family. Some only check their inboxes during set hours. Some prioritize answering inquiries for longer dates or from existing clients first. There’s no universal formula.

 
If you’re reaching out a couple of weeks ahead of time, I’d say give it 48–72 hours before moving on or following up. If you’re reaching out 48 hours before the date you want? Yeah, then the window to hear back is smaller. This is why I suggest pre-screening for clients who tend to be very last-minute; that way that logistical piece, which often takes so long, is done and dusted. So think ahead. Build those connections with people you’re genuinely excited about. Most of us want to say yes when we can but it’s a whole lot easier when you’re respectful of time, prepared, and communicative.

If you hate providers constantly checking their phone while they are in session with you, then you may consider being more patient.

 
I allow 2 days (sometimes more if it's a 'have to see' provider).

Context matters too...
I can see your suggestion to prescreen as a generally good thing, but I know at least one provider who responds curtly, or sometimes rudely, to any contact that does not request a meeting that same day or the next day.  That is her policy, as stated in her ads.  
So part of the context is the personality and policies of the provider in question.  
Or, in brief, YMMV.

At the end of the day, there isn’t one formula for all providers. Some are literally glued to their devices and tend to respond in under 24 hours. I would say even for those who aren’t glued, usually it’s no longer than 48 hours. Anything longer, there’s usually a reason. It can be a good one or bad one.

 
For me, what helps is what a provider puts in her ads/webpage. If taking weekends off from emails is your thing, cool. Put it in your ad or webpage, even cooler. If you’re busy and 48-72 hours is your standard, not a problem. Put it in your ad or webpage. If you set up clear expectations, then I’m not left guessing. Now, it’s on me to read your ads/webpage.

 
If I am contacting a new provider for the first time, time gaps will make me wonder. As someone who’s booked many providers, I have so many situations where I never got a response. I have ones where after a delay, there was another delay, and another, and another. I have had ones that ghosted me after agreeing to a date/time. No explanation. No further responses. And maybe they had good reasons. They at some point didn’t feel good about me? That’s their right. But it’s also why when delays that are longer than 48 hours, I start wondering if I will ever hear from her.

IOW, if the date you want is less than 7 days away, give her at least 24 hours. 1-2 weeks...48 hours. 2+ weeks...72 hours. Contacting on the weekend, or around a holiday, and I would add a day to the above.

I've had some women tell me that my email ended up in their spam folder.  

 
This happens sometimes with legit emails. I'd say two days is a good rule of thumb before trying again (weekends excepted). It also may be worthwhile to check her website to see if she's on vacation or some other hiatus. Of course, you may also consider reaching out by other means if you don't hear back (text/PM on social media account, etc)

 
I see one very well-known lady who posts her touring schedule on X and I generally email her right away for an appointment. I know I won't hear from her until about a week before her arrival even if that's a month out.

and 2 hours with an agency.   I consider them both a "reasonable" waiting period.   A provider or agency that takes longer than this will probably have additional problems running their business that contribute to NOT being able to give a timely response.