Early pre-booking is a great advantage for the ladies, but I've had mixed results as a client.
In my experience, the trend is that the farther out I schedule the less likely the date will occur as planned. In one case, given a six week lead, I confirmed two weeks out to a pleasant "Yes!", and then sent another a week out with no response. 48 hours before our date she finally responded to one of my daily requests for some reply to confirm with "Oh, I'm touring and out of the state right now, love". (Ah - I know what you're thinking, but her site calendar proved it).
In another case I'd set up my birthday party with three lovelies two months in advance, and spent half my birthday cold calling replacements for two of the three (I got a chance to meet them over subsequent months and they were well intentioned gals, all. And thanks, Heather Jana! You were the sweetheart that didn't let me down.)
All but one of my historical last minute emergency find-an-alternate expeditions have been in response to dates that I'd set up more than a month in advance.
Another dilemma is that you want to check periodically, but will need to balance that against looking like an annoyance or worse. Do you leave them alone and hope she really minds her calendar, or check in on occasion and maybe accidentally grow yourself a fire breathing, irritated bitch that's had just enough of you only two days before cuddle time?
Wait! There's more! - What if her reviews suddenly go into the shitter while you've already committed for that "perfect" date you scheduled? Alternatively, what if, while the calendar's counting down the days, another honey shows up on the TER board who's the spittin' image of your lifelong adolescent fantasy and at 'just getting started, introductory' pricing?
My advice, based only on my limited experience of course, is to set up the dates no more than two or three weeks out. If you like someone a lot well in advance, tell them you might want to get in touch in a few weeks and ask if you can pre-screen. A long lead time is a sweet thing to do for the gals, if you can, but it's not an advantage for the hobbyist.
Hope it all works for you, whatever lead time you choose. Hobby safe,
A.S.