When should I replace my clay bar ?

[rshan]

I have a Meguiars clay bar that's maybe 2 years old now. I've used it multiple times, and I keep kneeding it, and it keeps staying in pretty decent shape. So how do I know when it's time to get a new one ? My plan is to keep using this until I run out of the detailer spray that it came with.

Thanks,
Ray

[scrub]

Another brand recommends one bar can clean at least 3 cars before needing replacement.