I used to wash mine every 1-2 weeks. But now that it's garage and car covered (Evo 4 covercraft) when parked outside, I say once every month and detailed once a year.
Im paranoid when it comes to washing it. I have one microfiber towel for the hard top, one for the upper part of the body and a synthetic shamy for the lower parts. I buy a huge quantity of microfiber and regular towels and jack up the car to do a full chassis wipe down once a year. The engine is wiped down every month as well.