It all depends on the provider. How popular she is, prob the biggest factor. If she is very popular, and a touring gal, I'd book as soon as I knew she was coming to my town. For me, with local girls, I usually like to book about a week ahead, just to make sure I get the day and time I want.
Since you only play when you travel, if you know your itinerary, try to schedule when you can. Would suggest letting providers know your situation with the traveling, they will probably appreciate that information, as to why you want to schedule far in advance. Be sure to have your refs in order for them.
Good luck,
Swim