If you buy it in bulk 25 lb boxes you are ready to switch out in my opinion. First, try 2 tbl spoons of mineral spirits and 2 tbl spoons of Nu Finish in it. Run it empty for 15 minutes to distribute the liquid (with two used fabric softener sheets cut in quarters- optional. Pull them and replace with new used sheets every other batch.) This will shock you as to how the polish helps shorten tumble time and the mineral spirits/sheets cut down on dust.
+1 on this. I buy my crushed walnut from Harbor Freight its their blasting media it does a great job. You can also use lizard bedding as well from pet shops. You can save a bunch of cash that way.