Shows with families as the central cast always seem to go too long. They'll try to introduce a new character (cousin Oliver) or do something completely wacky (Roseanne wins the lottery).
I'd say these all went way too long:
The Brady Bunch
The Cosby Show
Little House on the Prairie
And as much as I love Spongebob Squarepants, I think he's getting close to the point of oversaturation and redundant episodes.