Well, if you want to keep on reusing it then it's up to you guys.
What I would say though is that the dirt doesn't 'disappear' anywhere it stays within the clay. the more you use it, the more likely you are to get hairline scratches in your paint.
I'm certainly no expert, but consider this pic...
It's the A4 I sold to buy my Mini. At six years' old, and 114,000 miles I'd say it looks pretty good: |