Once any light dirt is seen in the bar surface, knead and reform the bar so that a fresh portion of the bar is in contact with your cars' paint. Mould it into a good shape, about the same as how it was when it came out of the box - animal and car shapes are nice, but a shape that fits in your palm is best.
If you drop your bar of clay on the ground, it's all over red rover. Toss it out. Don't take any chances, If it's impregnated with grit it will scratch!