I don't know how long it takes them to open, but in my tests with Shockwaves, Rocket Steelheads and Rocky Mountain Assassins (once I got the Steelheads and Assassins fixed where they would open properly[>:])...

They all punched little bitty holes in the front side of a water-filled 1 gallon milk jug and they all made full cuts on the back side of the milk jug at 290 fps. 290 fps = 3,480 inches per second. With a milk jug being some 6" across, you're talking about fully opening within 17/10,000ths of a second.

But I'd imagine they'd open somewhat differently when contacting skin and flesh instead of plastic and water and at different speeds.