Sky... sorry, but that is a super load of Yak drool.
Over the years we've put together way more seven day cruises than longer ones, and ALL our group cruises have been meant for EVERYONE.
March 2012 we had a group on the NCL Epic out of Florida (with Chef Matt from Hell's Kitchen 4). We also had a version in 2011. We've had a CM group out of Galvestion in the last couple of New Years, as well as a couple of the Carnival "Blogger's Cruises". We've had a group on Allure of the Seas from Florida, that we had to cancel because no one was booking.
The majority of the seven day cruises we've done have resulted in smaller groups; like last years Epic where it was under 30 people.
With the history, since we began CM groups, we learned the cruises which were 10 days or longers, simply were considerably more popular (resulting in larger groups) than the wk. long cruises.
Part of the problem is the cruise lines group policies have made it more difficult, with rules, where they begin to take back unsold cabins within 30 days of establishing the group etc.
With logistics of a "site cruise", we do need longer to build a group for a specific sailing.
Another part of the problem is it seems a lot of cruisemates want the ship to "pick them up at their door".
When asking people what they might want to do as a CM group cruise, people tend to say they want them done on cruises on which they are already booked ; or everyone wants to choos a specific date... the only problem, the dates are usually all different