I've done quite a few RBs and have found that the ideal notice is no more than 2 to 3 days. I give the age range I'm looking for, the location, and the max I want to pay, and that's it. Have had as many as 20 responses, and as few as 5. There's always a handful of girls who take no notice of any of these requirements, but I've found the odd gem too.
And also found some extraordinarily good deals, or apparently good deals, with girls claiming they go out for 250 quid, saying they'll take my 120, or, if I'm feeling tight, 100 quid. (1 hr outcall.)
Fair play to them. If they haven't got much on, there's nothing wrong with a bit of yield management. Hotels and airlines do it all the time.