Received oath on Friday at Scarborough office.
Arriving 30 minutes before the scheduled time is more than enough. In fact, you don't actually enter the room until probably 15 mins after the scheduled time. They check your forms once again (PR card, original Confirmation of PR and the forms you sign when you receive the oath) before entering. One guest can accompany you inside (they can be in line with you). If there is room for other guests after everyone has entered they will let them in. If not, they have to wait outside.
As for the ceremony it takes about an hour. I would pay for 3 hours of parking to be on the safe side. The clerk and judge give you simple instructions and you say your oath and sing the anthem. Then you receive your certificate from the judge. It's a fairly informal ceremony. The judge was nice and making jokes to make everyone feel comfortable and happy. At the end, you can take a picture with the judge if you wish.
Good luck to everyone else!