Were you using Adams claybar and that wasn't working to your satisfaction, or were you using some other brand? I've never had an issue with Adams claybar, even on cars that were pretty neglected. Anyway to answer your original question, I use a piece of clay until I can't expose a clean side then throw it out or use it for glass. I can get probably 6-8 cars out of one claybar depending on the condition of the paint. Also, keep in mind that if you use a more aggressive clay, you may induce some marring in the paint which will require machine polishing to fix.