When I've been, I've booked in advance, the rotas usually appear on Thursday for the week starting the next Monday, but 24 or 48hrs should be sufficient unless you want a very specific timeslot.
Keep an eye on the rotas for any changes, as things can swap around sometimes, but if all is well, you then need to re-confirm your booking on the day between 10 & 10:30am and you'll be told if your girl is definitely confirmed for the day and the address details.
You could just book on the day, but you run the risk that your desired start time is unavailable.