While crossing the Atlantic or going from Florida to Alaska is a long trip, there are some shorter repositionings. The Disney Wonder and Emerald Princess for example, have 3 and 4 night sailings that are part of repositioning sailings sold as separate segments. If a ship is heading from Florida to Alaska, she'll likely make a stop in California after 7-10 days, and some guests may choose to board in San Francisco and disembark in Vancouver, for example, before the ship begins a season of Alaskan sailings. In fact, last year, we sailed on the Oceania Regatta from San Francisco, stopped in Astoria, Oregon, and stayed on for an Alaskan sailing before disembarking in Vancouver. It was fantastic taking a somewhat less than traditional itinerary.
Sailing past the Golden Gate Bridge on Oceania Regatta - May 2015
More than just distance makes some repositioning sailings long.
Looking at the Norwegian Jade on September 25th 2017, you probably aren't surprised that it takes 11 nights for her to make the long journey from Southampton, England, to Ft Lauderdale - but then why can Cunard's QM2 do this multiple times a year in only 7 nights? The short answer is that going faster burns more fuel. For some lines, like Cunard, passengers are going specifically for the Transatlantic journey, and they're willing to pay a premium for it. Therefore, Cunard wants to fit a good number of these crossings on their schedule, but they still have to maintain a balance, or else they'd make the crossing in only 5 nights, as they're well capable of doing. Most guests looking for a Norwegian cruise however aren't looking to cross the Atlantic, so the cruise line can't fetch as high of a price as they'd like, and cutting fuel helps their bottom line on these sailings. In addition, it isn't terrifically uncommon for some more substantial maintenance to be done during these longer sailings, though generally nothing disruptive to passengers of course. Sometimes this work relates directly to the new location out of which they'll be sailing, like when Royal Caribbean converts Rita's Crab Shack into Rita's Cantina on Brilliance of The Seas as she returns to the Caribbean from Alaska.