I suppose it depends on how contaminated each car is that you clay. When the bar becomes so soiled that it's still dirty after folding and kneading multiple times, then it's time for a new bar. If you are claying all new cars, ONE bar would probably last quite a while. But, if you're claying dirty, unkept cars, then one bar may last just only every few cars. It just depends on the condition of the cars and the quality of the bar as to how long it will last...IMO.