To get the "dust" out of your media, run a used drier sheet in the media for a while, it'll capture and hold the dust. Do this every now and then.
I've tumbled one heck of a bunch of brass in my media, can't remember if it's corn or walnut, but I haven't changed the media once yet. Just this summer alone, I've shot around 1500 rds., about the same last summer, and tumbled every last one of the brass before reloading.
I also add a splash of Flitz to my media every few batches also. Seems to speed up the process.