What is cool down on breaking a shield?

If you break your shield by scouting or attacking another player then the game imposes a cool down timer before you can place another one. The length of time will be 1 minute for each castle level.. So if your castle is level 20 then you must wait 20 minutes.

This was added to stop big players from porting to an area to attack bases then shielding up and waiting to attack again later.

The time depends on your castle level