I have some that I got for Christmas that have been through at least five cars on both a PCXP and a Griot's. They look a little torn up, but they still work..... Make sure you are cleaning your pad regularly and if you aren't satisfied with the cut/finish try the Kevin Brown Method.You are basically making two passes with the same amount of product as one.
What Jonathan said above. I had some 5.5" CCS pads from last summer that have probably polished out 20+ cars. Keep them clean and you should get a TON of life out of them. If they start to get pitted and torn, you should probably throw them out as they won't give a uniform polish at that point.