Anyone know how the +1 free locker works?

How exactly does the "watch an add for +1 free locker" work?

As in, how many uses do you have before it goes to the 8 hour-recharge? I have the impression I've used it at least 5 or 6 times before it had to recharge, but today I think it went on cooldown after only 2 uses.

Does anyone know exactly the mechanics of it?