http://www.humanbenchmark.com/tests/reactiontime/index.php
Human reaction time on average is 160-240 ms.
There are 60 frames per second in Brawl.
One second is 1000 milliseconds.
One frame is 16.6 milliseconds.
Hit boxes on moves typically come out on frame 7 [116 ms] and kill moves typically on frame 15 [249 ms].
Humanly speaking, most should only be able to react to moves that are 12 frames or slower; however, there's more to reactions that just raw frame data. Everyone has a 'bubble' around their character for their spacing. They know how close their opponent has to be to attack, and once the bubble is invaded, they begin to counter the attack. So by the time their opponent has began to think 'I am attacking now' the defense is already starting. This is why changing your tempo and spacing can lead to combos and instances where something that shouldn't work, does. Beyond this, people have conditioned themselves to play the game on a subconscious level [which plays into the bubble spacing of each character.]
Frame lesson over. >>