I'd agree with what mac_tlc said. On both of our cruises, we were through security at MCO by 9:45-10:00am. We took the
DCL buses both times. We usually got off the ship at about 7:45.
(by the way mac_tlc, we probably passed you between port and airport, as we sailed 12/1...)
BUT.....
On occasion, the ship is late getting back into Port Canaveral. Once due to a medical emergency, the ship didn't leave
Castaway Cay until about 9:00pm, and thus was about 3 hours late getting back into port. I think recently one of the ships stayed with a small craft in distress until the Coast Guard arrived. If something like this happens, you'll most likely miss your flight. It's very unlikely, maybe a 1 in 10 chance, but keep in mind that it is a possibility. You have to weigh the savings you'd get vs. the potential hassle you'll have if the ship is late and you miss your flight.
This is one of the reasons I book air through the cruise line. If the ship is late, they will take care of re-booking your flight, and it doesn't cost you anything. If you booked your own air, then you're on your own. I just think of the extra cost as being spent on peace of mind.