I can't tell ya the last time I washed it, cause I forgot.
Neither of my trucks EVER got washed. Old trucks with big tires look better dirty anyway.
My DD, The paint is rusting and falling off anyway, so I could care less, the dirt kinda hides all that.
When I get my new truck, it'll be either after I come home from off roading or once a week, which ever comes first.
EDIT: I don't know what everyone's car situation is, but in the winter, it is very important to wash your car. When they put down salt and dirt to break up the snow it also gets caked on under your car, in the wheelwells and on the body. After being under there it will begin to eat away at the undercarrage/body of the car, destroying your baby and it's potential resale value. If they salt/dirt your streets and highways, I would say go at least once a month starting soon after the first salting/dirting or the roads and get the full service (whatever includes getting the underside cleaned and "coated").
Just a friendly tip on car maintance and protection from your resident car guy, Clutch!