I have a few and they seem to last forever. I run them on my surface grinder as well as use em to clean up my grinds and surface prep. etc. What I have done is take a piece of scrap steel. and use it on a flat platen, I sort of lean on the belt running the steel back and forth a few times. changing corners on the scrap to a fresh hard corner a couple times.. You dont want to push To hard. or you will remove all the abrasives yet harder then you normally would grind.
I also think its worth mentioning, IME these belts dont like water. so work them as dry as possible.. Also, never leave one of these on your grinder they tend to stretch out and wont run true.. Last. I never use these for anything other then steel.