The time to be back on the ship varies from port to port and from ship to ship.
For each of the Port Days, look carefully at your daily Fun Times. It will tell you things like where the gangplank will be located for getting off the ship and what time to be back on the ship.
I am a Cruisaholic; "I'm Not Dead Yet"
Carnival Inspiration, January 17, 2014 Long Beach, CA *** Carnival Liberty, March 22, 2014, Miami, Florida *** Carnival Miracle, September 10, 2014, Seattle, WA *** Carnival Freedom, March 21, 2015, Fort Lauderdale, FL