I have cruised on both CCL and RCCL, and all cruises were 7-8 days in length. We did notice that people with children tended to go to the buffet on formal nights. That being said, no one forces you to wear formal attire to dinner on those nights.
On Adventure of the Seas last summer, there were 6 of us at our table, and 2 of them men didn't even wear ties on formal night. The other two men wore blazers and ties, and the other woman and I wore cocktail dresses. We did notice that there was a family at a table across the room, and they had on very casual clothing. . . . .
I also noted that while the cruise lines ask you not to wear shorts in the dining room, they don't stop you if you do. There were a lot of people who walked in in shorts and t-shirts.