De-leveling Buildings with Ballista

[vSpot]

What is your experience with destroying an opponents building with a ballista?

The opponent's average building level is 18 and I would like to know how many times I would have to attack them to reduce each building to rubble.

[Joker79]

Each building is different. I have obliterated a few to nothing but a town hall by sending waves of 100 ballistas aimed at a different building with each wave until there was nothing left. It takes many attacks for it to work.