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.
I agree with Jim C.: three hours at Pompeii is just about enough, unless archaeology is of particular interest to you. We saw Pompeii, Sorrento and Capri all in one day. It was a long day, but very well balanced.
That can be done, but requires good pre-planning. There are websites available for schedules of ferries to and from Sorrento/Capri and Naples. I would suggest taking the train to Pompeii, spend a couple of hours there and then continue on the train to Sorrento see it, grab a ferry to Capri and then go from Capri directly back to Naples.
Leave buffer time at the end and buy your return to Naples ferry tickets as soon as you get to Capri so that you know you have a seat and will get back. It will be rushed but its doable provided you get off the ship ASAP in the morning and the ship doesn't leave until the evening.
It's easily doable in one day but I feel compelled to echo what the others have previously stated. Do your research before the trip. Know what you want to accomplish and have a gameplan prior to the cruise. Study the history behind Pompeii and it will prove to be a vastly more rewarding trip. Hint... Study the roles of Pliny the Younger and his uncle Pliny the Elder in the Vesuvian history. It sheds light on the true impact and devastation from an eyewitness point of view.