How long I tumble depends upon how long it takes to clean the brass. Some brass will clean up in an hour or sometimes it will take 6 hours. I've found the more you shoot the brass the longer it takes each time to clean it. BTW, I'm not looking for super shinny brass, I'm only interested in it being clean.
As for media, again, it depends on how dirty the brass is. I've used some media as long as a year but I do use cut up USED dryer sheets to clean the media. You would be surprised how much dirt the dryer sheets remove from the media. I use a cut up sheet about every 4 uses and that seems to keep the media fairly clean for a very long time.
This is just what I do. I'm sure everyone has a different way of doing things...