There is no appointment necessary, so you can either just turn up and see how many people are waiting, or you can ring them before going down and they'll let you know how many people are waiting. I've been there twice now. First visit, I waited about 45 minutes and the second visit waited 15 minutes, so it varies, similar to a normal appointment, where you can wait this long/longer as well. I found the service to be very good and the rooms are very modern and well setup. You can also get blood tests within the centre, which is handy.