So, what part of my "No BUMPing" didn't you understand?
:nono
The answer to your question is highly variable... which is why you're not getting an answer. When the pad begins to visibly wear (chunks missing), it's time to toss it out. The pads do wear differently by grade. Plus, the pads will wear differently based on the products you use (compound vs. glaze). Machine speed is also a factor, as is how you use it.
db