It depends entirely on the itinerary. Some ports, like Livorno for Florence, or LeHavre for Paris, the ships stay much later because of the time necessary to get to the "major sights". Ships often overnight in Venice, where that's included in the itinerary.
If you look at some of other lines, I think Azamara Cruise Club, and Oceania are now featuring many more overnights, and longer stays in ports of call.