Most of the mainstream cruising I've done lately have had a lot of sea days (at least 8 over 2-3+ weeks) since 2 out of 3 have been primarily for intercontinental travel.

Anyway, breakfast, lunch, sometimes tea (horrible tea on Holland America ships) and dinner in the dining room will burn up at least 3 1/2 - 4 hours of a day. Gym and or pool another, 1-2 hours, trivia (1-2 hrs a day, earns me lots of pins), reading, lectures if any.

Have an upcoming 17 day cruise with no stops, and no entertainment other than the ship library/lounge.