It lasts a long time. If the brass comes out clean ... it's still good. I usually cut up a "Bounce" sheet (that's Bounce for your clothes dryer) into four strips and throw it in. It seems to collect the grit that would otherwise just keep circulating in the media. I use a large Dillion vibratory brass cleaner not a tumbler style but the bounce sheet will still work for you, you just might not need to use the whole thing, cut it into four strips and just use two. How dirty it is when your done will determine if you might want to use more of it or not. It's cheap in any case, so no worries. What are you using? corn cob or crushed walnut? Are you putting any polishing stuff in with the media? Keep in mind that the brass only needs to be clean, shiny is nice but not a requirement. If you're new to this, I would suggest NOT de-priming the casings before tumbling or you'll be picking media out the primer pocket. If you do clean your brass without primers, the next time you're at the Dentist, ask for any of their used cleaning instruments, great for picking media out.
