We've been cooking/prepping a lot more food, so we've upped the cleaning schedule to once every two weeks. Lately we've been doing it once a week, at the same time we scrub down the stove.
That's a lot more frequent than our previous schedule! Which was, whenever we opened the microwave and noticed something yucky. (Or at least, that was my system; I'm sure Mrs. Bee had a more systematic approach I was totally unaware of.)
How often do you scrub down your microwave?