I typically book same day (probably 70% of my sessions so far). Today, I texted a local lady I hadn't seen before and was there an hour later. My shortest was a couple weeks before Christmas. I wasn't planning to see anyone that night, but as I drove by my favourite's apartment (she works a short distance from my house and is pretty much on the way from here to anywhere else), I pulled into the school parking lot across the road and shot her a text. 10 minutes later my head was between her legs.
I've booked a few a day ahead and a couple several days ahead. Since 2 days after my first meeting with Jessica on Boxing Day, I've been booked to see her on her return here in mid-February, so that would be 7 weeks in advance. She was even nice enough to modify her dates here to make it work for me, so I hope I don't get the cold from hell or something that week.