I love to cruise. The longer the better but there are some things to consider. We did an 11 day cruise from Hawaii in April. Around the islands was great. I looked forward to the 7 days cruise back to the mainland. I planned to lay out by the pool every day with a good book and just soak up the sun. Boy, was I wrong! Within 5 miles of shore it became so windy on deck that you could not walk. Once out to sea it also became quite chilly. It's a good thing they had lots of activities planned as we spent most of the week inside. Only one day was it nice enough to lay on deck.
Also your longer cruises will cater more to an older/retired crowd as they have more time for cruising. I did notice this on our 11 day cruise. My husband and I are in our 50s and I felt like the younger generation.
Younger cruisers tend to only have a week.