Valad, the media will last a lifetime. The number of cases you can tumble will be limited by the capacity of the tumbler used. Mine is a Thumbler's Tumbler and it's rated at 15 pounds gross, but I generally push it a bit to about 16 pounds. In a previous (above) post I told how I get a bit more brass into a load. I couldn't say how much more brass I'm tumbling than the instructions would have you doing, I haven't counted them out. All I know for sure is that I'm not tumbling as much water as the instructions direct and I'm making up the weight difference with more brass per load.
