That really depends on which ship and what the actual itinerary is. Anywhere from 7 nights with 4 ports and cruising a glacier bay on up. HAL, Princess and now RCI/Celebrity even offer pre or post land excursions as part of a package including hotel and land transportation. It is a lot of information to shift through. If this is your very first cruise, start with what you absolutely must see in Alaska; i.e. Skagway, Juneau, Ketchican, Glacier Bay, Misty Fiords, etc. If you must see wild life, then you would want to include a tour inland where you would have more opportunity to spot bears, goats, etc.