I've only ever booked in advance ONCE, and it was for a popular lady who made it clear on her in her profile that she was typically very busy and that PM bookings done through the site would be given priority. Other than that, nearly all of the bookings I've ever had were of the almost immediate variety, in that I'd ring, ask if they can meet within the next hour (or sooner), and then go from there. If they tell me I'll have to wait anything over 2 hours, I tell them I'll try again another time. Only had 1 ever that kept me waiting 2 hours, but it was well worth the wait!