Used dryer sheets or paper towels cut into strips will remove a lot of the free residue from tumbling and extend the life.

One thing that can greatly impact the life of your media is how dirty your brass is when you start, what additives you incorporate, and the level of shine you seek.

I normally use fine (20/40 sieve size) walnut hull media with a cap of OMS (odorless mineral spirits) for basic cleaning of brass after firing. I use the same for removing case lube after sizing bottleneck cartridges. This will clean very effectively but not put a super shine on the brass.

If I want bright shiny brass, I use fine corn cob and either add some NuFinish or Frankford Arsenal Brass Polish. Either will shine the brass like chrome.

Your estimate of 1600 rounds per pound is lower than my normal experience. But it seems to me if the media is will used it actually cleans faster than new media. But it depends a lot how much "filth" you want to deal with.