I like to get there at least 2 days prior to sailing for foreign destinations. That will give you time to explore the city you are leaving from. The cruise itinerary usually does not give you time to see the starting or ending city.
It will also give to insurance time in case your flight is delayed or your bags are delayed. In two recent cruises my bags were delayed 1 day (Baltimore to Vancouver, BC) and 2 days (Dulles to Cambodia). In both cases they made the boat, but it was close.
