On a cruise, there is something for everyone, however, too much of a good thing might be TOO MUCH.
My wife and I like taking cruises very much but we don't ever just lie around.
We do a lot of dancing (every night), we play bridge (cards), we go to the gym nearly every day, we walk the promenade deck when the weather permits, we take classes, and we also take advantage of the shows, the games, dressing up, etc. We also visit whatever ports of call and try to do as many things as possible within financial constraints, sometimes even taking larger plunges for very unuasual opportunities.
We have not found a limit in terms of days, but our longest cruise has been 15 days.
Each of us is different. I cannot just lie on a beach in the sun, even for an hour. I either read a book,sleep, or I must walk, play frisbee, etc