It depends on the external factors to which your car is exposed.  Salt, sand, or other corrosive matter should be washed off immediately.  The frequency of regular washing or waxing will vary depending on where your car is parked: in a garage, in the sun, under the carport, under a tree, or in areas of acid rain.  Also, remember rain does not wash your car.  Your car should be washed anywhere from once a week to once every two weeks.  Applying on-line waxes (Rain-X, Simonize Hot Wax, Poly Shell 1000) in the car wash tunnel definitely helps prolong and protect you car’s finish.  Express Hand Waxing should be done every two to three months.  Preventative maintenance will save your car’s paint. Don’t wait till oxidation and dullness take over.