Some react faster and some react more slowly, but the average driver's reaction time is one second.
Readiness for action, a defensive driving style and high risk awareness combined with a lot of experience are crucial for a good reaction ability.
Fatigue, inattention, alcohol, drugs and illness can impair your reaction ability and thereby prolong your reaction time.
It is your reaction time and the car's speed that determines how long the reaction distance will be in a braking event. The reaction distance is the distance the car travels from when you discover an obstacle or a hazard until you apply the brakes.