I use 3/4 walnut & 1/4 corn cob with a capful of Nufinish. Also cut a used dryer sheet in 1/4's for dust. The dryer sheets get tossed after every batch. I run about 600-700 rounds on a capful of Nufinish at 250-300 rounds per batch. I toss the media mix when the corn cob is charcoal colored. I have run about 3k rounds through in about 4 weeks on this media mix. Will probably change the mix before I run anymore brass. I find that the Nufinish helps to lube the .40 Brass before I run it through the GRX die. Crushed walnut and corn cob is cheap at Petsmart, 25lb bags at a time.
Edit: I also bought a plug in timer for my tumbler. Now I set the timer, fill the tumbler and check it when I think about it. I can set it to start right before I get home from work or can set it to go off sometime during the night and don't have to worry about it.