I did one grand cruise a couple years back - One week on Carnival Victory's first Caribbean cruise, then jumped over to the QE2, Queen's class for a week (talk about shock - inside Carnival to Outside Queen's Suite - mind-boggling!), then jumped over to Ocean Princess for 2 weeks. Went from bottom to top to middle, but had a ball. However, after the 3rd week, I was ready to get off and my sea-legs lasted almost 2 weeks (ugh), plus I started to get some sort of creeping crud that was going around the ship. I've found that the longer I stay on a ship, the higher the possibility of getting the "cruise-cold." One is stuck in a fairly confined structure and germs are way too easy to catch.
Two weeks is enough for me - doing two next Feb on HAL Zuiderdam - can't wait.