Howdy,
In the winter I clean mine a couple of times a week because UDOT uses straight salt and alot of it. In the summer it's every Sunday night. I have to do it that often because I commute and it gets real gunky by the end of the week.
I'll also give it a good cleaning before centuries and races, it's pretty much an ego thing, I don't want to be seen on a trashy looking ride.
M.R.