I suggest you put some balls in a glass jar, 1/2 full & place it in the media and turn on the tumbler. The vibrations will rotate the jar a certain direction. Turn the jar end for end to whichever way is necessary for the jar lid to always be trying to tighten. If placed the opening direction, you'll be fishing balls from the media, even after a short period of time. Fellow who moly coats by tumbling mentioned this several years ago.
I don't think it matters how much time, as long as it's enough. I suspect you're trying to hammer down the sprues. I would think 1/2 hour might be enough. The balls won't get much if any smaller to a certain point, then that's it.