When I first got mine it needed a bloody good blowing out after a week, it was full of all sorts of clackit, about a month later I noticed a blob in a landscape piccy and got rid of it by manually doing a few "vibrates" in a row (which I was quite impressed by), over the last 3 months absolutely nowt, nothing, nada, despite a good few lens changes in all sorts of environments.
I've got mine to clean on shutdown and it seems effective, the sensor stays cleaner than my old D50 did.