It depends. If your car is exposed to corrosive materials like salt and sand, it should be washed often. The same goes for vehicles in areas with high humidity. Moisture attracts contaminants and promotes a variety of chemical reactions that can destroy your car's finish. Frequent washing is essential to the life of your car's chrome and painted surfaces. In seasons or climates less conductive to corrosion, a wash every two weeks should be adequate. In areas with acid rain, a wash every ten days is advised. Of course, these estimates may vary depending upon whether your car is kept in a garage or out in the elements.