It really depends on what you want to get out of it and how fast you can visit such things.
We went and saw enough for us (which was quite a good amount of the place. With the audio guide to listen to we walked probably about 2/3rd of the open tot he public areas. There is a fair amount of the audio guide information that starts to sound the same after a while so we didn't spend as much time listening as we could have. It took us about 3 hours. We saw The Herculaneum (sp?), Pompeii, and took the train down to Sorrento and had some food and came back to the ship in the day we were in Naples.
BUT, having said that, there is probably much that we missed and will need to go again (good excuse to go back) But we wanted to see all three places, so we did it. Also keep in mind we are healthy people and normally do a lot of walking.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007