There can always be a glitch, but on our Freedom cruise they called "express departure" promptly at 6:30am, which was the time posted in the debarkation flyer they handed out the day before. These passengers had to carry all of their luggage with them - no exceptions. I know a few people who managed to get on earlier flights out of MIA and were airborne by 8:30 or so. These were the lucky ones on our particular debarkation day.
If you need to have your luggage moved ashore in the usual manner, then you will need to get beige, lavender, or gray tags. These colors are scheduled for 7:30am, but can be called earlier. Of course, on our cruise they screwed it up terribly and called colors over an hour before the schedule and as a result our luggage didn't show up for over an hour.