I have left the blade in the tumbler for many hours at a time. Check it periodically, mostly because you are curious that way,
( I was). It should be fine. One note on the corn cob, there is a regular and fine cob mix for the tumbler. I got my media at Bass Pro Shop. The walnut will clean the thing pretty well and leave a little shine. When you put it in the corn cob, let it run a little while and then add some of the metal polish. I have a theory that the media needs to bang the rough edges off of each other to really polish the metal. It just seemed that way yo me. I know when I added that first bit of polish the cleaning accelorated a good bit. I think you will like the results.