Supercedure: When a failing queen is being replaced. The old queen is still there.
Emergency: When a queen goes missing or is killed and the bees are trying to replace here. The hive is queenless.
Swarm: When the bees are intending to split the hive and they require extra queens to do so.
If you have a true supercedure cell, then you have a queen. As long as you have a queen, it won't matter if you destroy the cell. If you want to requeen with purchased stock, then you need to remove both the queen and the supercedure cell. 2 hours is the minimum time for them to realize without a doubt that the queen is gone and to shift gears where they will accept a new one. (they can tell the queen is gone in about 10 minutes which you can tell by the dissonent roar from the hive). I would not go more than 24 hours.
If you have an emergency cell, I would just destroy it and give them the new queen. You don't need to wait, but 2 hours would be the minimum.