Looking back at our cruise history, there were a number of 7 day cruises and just one 8 day. I can't remember that the 8 day had any particular added benefit beyond, as they say in Les Mis. . ."one day more."
There is, in my view, a major sea change (sorry) once you break the 10 day barrier. Most of our cruises now are 12 to 16 days, including frequent crossings, and we find them generally much more rewarding. It's not only the extra time onboard. There's usually an entirely different vibe on longer runs, and I think there are a number of reasons why.
Subjective view of course. Your experience may vary.