Carnival attempts to get everyone off the ship by 10 or 10:30am, so they can start letting the new passengers on by 11:30 or noon.
Since you are referring to a cruise that departs at 10pm, it would also depend on when they allow them to board. Since you are taking the same cruise, look at when you can get on the ship at the beginning of your cruise, and expect to be off 1 hour prior to that upon your return.
DS - 
Cruise Specialist
ShipVacations.com
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