Well, in the movies, the guy hit by the tranq gun always gets to stumble drunkenly, clutch at the dart and stare at it in slowly dawning horror, collapse to his knees, and try to crawl away or shove his plot-important CD-ROM into a sewer grating or something as the bad guys close in. Usually, the bad guys have the chance to get to him, grab him by the arm, and say, "Mister Hero, you're a difficult man to find," before they blur out into darkness (and then our hero wakes up tied to a nuclear missile in the middle of an alligator pit).
So I didn't think that immediate unconsciousness was in keeping with usual cinematic terms.
In realistic terms, I have no idea at all, although my guess would be that it would take a bit of time to get into the bloodstream, no? I don't know that it would take a full minute, but I didn't think making a 30-second house rule was worth it.
Das Darke, what were the weapon stats in Urban Arcana?