I was just wondering.
What happens if you don't add another super in time and the bees have filled all there frames but have no where to build more comb etc and the queen has no where to lay?
I would think if the hive is that full that they may have already swarmed and remaining field bees are filling comb.You may also have a virgin or young queen in there with no place to start repopulating the colony. Add an empty box below the full one.