The 7 day was too long primarily because the seas were very rough (30 feet!!!). But looking back, other than the weather, 7 days just seemed like a long time for both Mr Muush and myself. Now that we are cruising experts

(we have taken 7, 4, 5, 7, 7) and just love cruising, we don't see ourselves taking a cruise longer than 7 days. We might be the rare ADD type of cruisers because so many people like to take long cruises.
With
DCL, the cruises are so dang expensive, a 4 day for a first cruise is easier to finance, you can pretty much assume that it will be a great time and that you will be sad to leave the ship.
As far as not enough to do, we are not typical cruisers. We cruise for the actual being on the sea experience rather than the shows, meals, and activities that are going on. We enjoy the ship, so when visiting ports, we think great! everyone will be gone and we can be on a nice quiet ship!
Our favorite thing to do is to sit on the veranda and feel the sea breeze going over our toes. And if our veranda is not so big, sitting out on the promenade deck is our second favorite place to hang. This past 7 day DCL, we did not go to one show! We had seen them before and just didn't want to see them again.
Sorry for the long answer, I probably need more coffee to get me moving physically and stop dreaming about cruises!