Mine says it goes to dark in 0.00004 seconds, and I don't notice even a moment of brightness when I strike an arc. I would guess that 2 ms (0.002 seconds) wouldn't be detectable either.
But see http://www.mig-welding.co.uk/forum/showthread.php?t=30
Nial says at the end "I don't like it. When you strike the arc there's enough of a delay in the lens darkening that you can just pick it up and this puts me off."
Maybe he'd know how quickly his reacts. Could be that I could wear his and not notice?
I tried Rob's helmet and there was definately some arc light before the darkening cut in. And I'm sure I could still see that arc after welding. I'm sure if people sell this stuff it's going to be fast enough, but I did find it spooky.
Is there much difference in reaction time between different masks?