It is a little strange that they left out casting time twice, but I'd agree with Dandu on this one and rule it a standard action to cast.
I usually try to avoid using the descriptive flavor text as a basis for for making rulings, since it is often not consistent, but even if you wanted to go that route, in this case I still don't think that the flavor text says what you want it to say.
From the Spell Compendium, the text says "You feel a spectral force form near you, as if some intangible shadow being stood nearby. Upon choosing the target of your spell, you sense the shadowy force depart from your side, moving as fast as thought toward the creature you designated as your target. You see the target creature move as if attacked by an unseen foe."
The way I read it, the "moving as fast as thought" part refers to the spell taking effect, not the casting time of the spell. If it was referring to casting time, it would have to say something like "faster than a thought you cause a shadowy force to...". I read this text as saying: 1) cast spell during which time you feel shadows gather - then 2) finish casting by designating target and watch the shadows moves fast as a thought to do whatever.
So in the absence of any other information, i would still call it a standard action cast.
