Dark Seal

Recast time will be reduced from fifteen minutes to five.
Additional merit points will no longer reduce recast time, but instead reduce casting time for dark magic spells by an additional 10% per point.

My question here is simple; is this only under the short duration until you cast a dark magic spell and thus, effect that spell only? Or is there a secondary duration under which the reduced casting time is active.

Simply put; is this yet another adjustment too good to be true?

Why not just give Dark Celerity as a job trait already? Start it off as like 2% for Tier 1 and let it go up by that until like Tier 5 if you want it not to be unbalanced. I'm sure we'll all be happy with a 10% casting time reduction for dark magic.