The life cycle of clay is limited. The best way to tell when it is time to get a new piece of clay is to check the color and texture of the piece you are working with. As you rub the clay bar across the surface of your paint, you are picking up bonded contaminants. After some time, you "fill up" the clay with these particulates. You can tell when the clay is "full" because it will have a darkish color and the clay might begin to feel rough to the touch. Another time to switch to a new piece of clay is if you accidentally drop the piece you are working with. When clay hits the garage floor, it will pick up large dirt particles from the floor, which can in turn scratch your vehicle if you continue using the clay. When your clay bar is "full" or has been dropped, we recommend you throw away that piece and utilize new piece of clay to avoid scratching the surface of your paint.