Corn cob media is gentler, I'm not sure it's a better polishing agent however since it all depends on what you're trying to polish. For most stuff I run in my tumbler (pinball parts) it takes a minimum of 24 hrs, usually 48 to 72 to really shine things up.
I think it's safe to experiment with a batch of common tokens for up to 72 hrs in walnut media, without any loss of surface detail or plating. At that point, they'll be as far as walnut will go, I'm not sure corn cob would add anything finish-wise to that type of material, it's usually for the really soft stuff like yellow and red brass, and aluminum.