I need some help with a problem I want to solve.
You have a gunman standing 50 ft away from a human target. According to google, the average bullet travels at 2,500 fps. If my math is correct (debatable, my math skills are kind of sad) this gives the target .02 seconds or 20 ms to react before the bullet reaches them.
Also according to google, The average reaction time for humans is 0.25 seconds to a visual stimulus, 0.17 for an audio stimulus, and 0.15 seconds for a touch stimulus (Logically speaking, I believe this is the time between the initial stimulus and the time motion is initiated, not completed). I'm not sure (could not find) the amount of time would take this average human to actually finish moving their hand into a position to block the bullet's path if it is headed directly for the person's heart. Edit: couldn't find numbers online, so I found a victim and measured him. He is 6 ft tall, his arm is about 30 inches long from finger-tips to shoulder, and the straight-line distance between his hand and the approximate area of his heart was about 25 inches (2.08333 ft). So let's assume the hand needs to cover a distance of about 25 inches. I also found this statement: "The human hand can move 150 MPH(.04 miles per second which equals 215 feet per second" here: http://highdeas.com/lifestyle/This_is_insane_but_I_cant_disprove_it (but there was no reference to say where the number came from).
Edit 2: I checked the number with an online calculator and it would seem that 150 mph is actually 220 fps, not 215.
Obviously, average Joe (and even exceptional Bob) are far to slow to have a prayer of putting their hand in the way of a bullet (and even if they did, the bullet would rip a hole in their hand).
What I want to know is if it is possible to determine how many times faster than normal human reaction time Superman would need to be in order to block a bullet with his hand.
So, from your data, it seems that the average human would take around $0.25s$ from the moment he/she saw the gun firing to start the movement.
Then, the same human would need at least $0.01s$ to move the hand in front of the heart, assuming that the hand can immediately move at top speed.
Therefore, average humans would complete their defence movement in over $0.26s$ after they saw the gun firing.
Superman would need to be 13 times faster to do the same thing in $0.02s$, since $\frac{0.26}{0.02} = 13$.
By the way, looking at a related scientific article we can see that the reaction times of young male adults is around $0.226s$, so it's close to the estimates in here.