I think about 0.6 to 1.0 seconds, when excluding instastabs, hiltslashes and other borky attacks.
My main problem with blocking with NA ping (a mere 115), NA 3, mostly which seems to be a worse server than NA 1, is not that I do not have the time to block, but that the server seems to constantly get my block direction wrong. Even when I block as quick as I can, which should probably still leave like 0.5 spare time, even with this ping, and that is worst case scenario. The attack goes straight through my block regardless. About 50% of my blocks simply fail even though it looks perfectly fine on my screen and with easily enough time to spare to compensate for the ping.
My point is that ping is not just delay but also synchronization issues, packet loss or something. Which in my experience with playing NA strat are much more troublesome than the delay when it comes to blocking. If ping only resulted in delay I guess that if you were a fast blocker you should be able to block pretty well with 240 ping. Human reaction is probably more like 200-300 ms in such a situation though. 170 ms is probably measured without any distraction and in perfect concentration and even then it seems quite a bit faster than average. With extensive synchronization issues however, blocking is probably neigh impossible.