How Often Do You Redecorate Your Sim's Houses?

Whenever the way it currently looks becomes distracting to gameplay.
"Hey, I don't have a toddler bed and now I need one"
"Hey, Grandma's room probably doesn't need to be kept the same way so long after her death"
"Hey, this couple is middle-aged and reasonably well-off, their apartment shouldn't look like the apartment of two penniless 20-somethings who think bar decor on the wall is funny"