I would say anything after 11:00. Disney recommends nothing before noon, but as long as you can get off the ship early you should have no problems. Though nothing is ever guaranteed and you never know if traffic will be a problem or not due to an accident or whatever!!!
We actually had a medical emergency on our sailing last summer that put us back in Port Canaveral very late. We did not get off the ship until after 10:30 am. Many people missed their flights.
MJ