I only set 1-2 outfits per category. I change them whenever the mood strikes me -- because I'm bored or for story/character reasons -- but usually it's not that often. I do always adjust their wardrobes after they enter a new life stage.
I've finding with Seasons I change their everyday outfits more often, because your everyday clothes in summer would be quite different than in winter, even if it wasn't hot/cold enough to trigger the special outfits!