I let mine tumble until I get the polish that I am looking for.
And that all depends on how dirty and tarnished it is before I load the tumbler.
If it is real bad I put it in the walnut shell first and check on it about every 15 min. or so until all of the dark is off of it.
And then I put it in the corn cob with polish run it for an hour or so and then start checking it.
I am not familiar with he metal medias I am still pretty old school. But it works for me.