How long is a piece of string?
Some I've encountered start the hour when you walk through the door, whereas most seem to start when you exchange paperwork so long as you don't take the piss. At the other end, some are happy to chat for 5-15 minutes after, whereas most want you to be getting dressed and heading to the door by the time the hour is up.
My advice would be to turn up, spend 5 minutes at most sorting out admin, be polite and leave on time.