I change my corn cob media when cases start coming out with black smudges or flecks on them that I attribute to boolit lube and/or powder fouling. Lately the life of my media seems to have been extended by the addition of 2 or 3, 2" or so square pieces of used dryer sheets and adding a couplea capfuls of car polish every 3rd or 4th tumbling session. The dryer sheets seem to suck up the crud that gets interspersed with the media.
The dryer sheets get tossed after each tumbling session.
When I do change out the media, I also clean out the bowl with hot water and soap. This removes the debris that clings to the sides of the bowl.