How Often Do You Redecorate Your Sim's Houses?

It varies, but usually once a life stage for the bedrooms/personal rooms (office, studio, gym, playroom, etc.) and once a generation for the common rooms (living, dining, kitchen, family) if I'm playing a Legacy. Basically when the next heir "comes of age" then I'll redo the living room and the main bedroom to suit them, and then rework the rest as it feels right.