I change it out when it takes more than a couple of hours to get pristine brass. Also, I use a 50/50 mix of walnut (from the pet store!!! Much much cheaper) and corn cob media. One capful of NuFinish (7 bucks at Wallyworld) per tumbler load. One more thing.... Use a old dryer sheet, cut in half or quarters. The dust from the Walnut media holds powder and lead dust and the dryer sheets seem to hold the dust to a minimum.
DaFadda

Thanks Padre!