I use walnut treated with Lyman case polish for the first cleaning/polishing. After I resize them, I run them through with untreated walnut. They come out looking like factory fresh, minus the little dings and scratches that come from firing, extracting, etc.
Total tumbling time is no more than two hours. I have cut it to half that, with little change in the final result, but normally do two hours just to be sure.
As a side note, I used a rotary tumbler for years, then got a Midway vibrator. World of difference. I may keep the rotary as an emergency backup, but, in my mind, the vibrator is the way to go.