I would recoment arriving between 11:00 am and noon. The lines should be shorter than if you arrive after noon, and you get on board that much quicker (and get in the fromt of the lunch buffet line, too).
We try to get there by at least 10:30. You get processed faster and by 11:30 you should be on the ship and go to lunch. 12:30 you can go to your room and check in. The later you arrive, the more people will be coming.