This varies widely, from Mediterranean cruises that are in ports every day to longer sailings in Asia with a handful of sea days required to cover the distance between ports. In the Caribbean, you may have 1 or 2 sea days on an itinerary and, generally, Alaska itineraries spend 3 or 4 days in port with 1 or 2 days of scenic cruising in-between.
Coming into port, ships generally dock right after breakfast, allowing you the morning and afternoon to take a shore excursion or explore on your own. Ships usually depart in the early evening, giving you an hour or two to rest up before dinner (although some ships do stay in specific ports, such as Juneau or Cozumel, as late as 10pm, giving passengers a chance to sample a dinner on shore). Some lines, like Azamara and Oceania, make a point of designing their itineraries with multiple overnight stays in port. And voyages to Bermuda, for example, typically feature 2 or 3 days docked in Hamilton before ships set sail across the Atlantic for the eastern seaboard.
This is where your own personal tastes will come into play: You can easily find a Mediterranean voyage that is packed with port calls, or one that is interspersed with sea days in between. While a week of solid ports sounds like fun, the truth is that it can be very exhausting. A nice mix of sea days can provide that much-needed down time to relax and recuperate without having to worry about FOMO: the Fear Of Missing Out.
On days at sea, the emphasis will be on activities (think bingo, pool games, and wine tasting) or, in some parts of the world exploring natural areas and scanning for wildlife. Big ships stick to prearranged schedules on these days, but on small-ship, soft-adventure-type cruises, days at sea can be unstructured, with the captain choosing a destination based on reports of whale sightings, for example. Some small-ship itineraries include almost no ports, sticking instead to isolated natural areas that passengers explore by kayak, Zodiac boat or on foot.