It is often suggested that you should wash your car every two weeks. But this can also depend on various factors as the bugs dust or road salt might get you to clean your car more often. Also, the climate condition of the area you live in and how often you use your car will determine after how long you should shampoo your car. As it does not have any side effects you can do it every week or more often. But that will be a waste of energy, water, and car wash soap as the car does not require cleaning that often.