MTB gets washed every ride and the drive train, cables, and pivot points cleaned and lubed, but then again, it usually really needs it!
Weekend bike gets cleaned and polished every few rides, it's my pride and joy so I'd much rather see it clean and gleaming in the shed than encrusted in mud.
Pootling around roadster bikes seem to have such effective mud guards that they don't really need that much cleaning (the low speed might help too!), so very rarely.
Commuter every fortnight or more often if the weather's filthy and there's grit on the road - this is the bike that sees the most miles, so the answer I gave. My wife's bike gets the same treatment.
I enjoy cleaning my bikes, and fettling around with them a bit at the same time, but there are only so many hours in the week!