0.261 seconds.
I don't really like this sort of thing either, since it's just a click... It doesn't account for input errors, mental pressure, animation similarities, etc., etc....
A Soul Calibur player from Hong Kong told me some years ago, that moves under 20 frames were basically going to be impossible to react to... Moves under 25 would be very difficult. I find I can block a 24 frame low if I'm lucky and "hot" that day.
Reacting to something 17 frames... would mean throws would be useless in Calibur. I've never seen that before.