Forbidden Power

Sorry if this has been answered already, but searching the forum for it is an absolute nightmare since all you get are build posts mostly:

So, does it only grant me a Power Charge if I spend 200 at once, or do I have a grace period for spending those 200 mana points? (Like, say, 4 seconds.)

Essentially, would this work with Cast While Channelling? I would be spending 200 mana often, just not at once every click.