I think audiboy has is right. It all depens on the size of the car, the cleanliness of the paint as well as the type of clay. Some clays are more abrasive then others as it is just a polimer and adbrasive (i think correctme if im wrong) and so it all depends on what you have. You will however know when its dead as it dosent work as well its hard to explain how it looks/feels but you will know