The formula for the time (in seconds, s) to fall at a distance d (in meters m) in earth's gravity is d=0.5gt^2 where g=9.8m/s^2 is the acceleration due to gravity. Then we get to

$$t=\sqrt{\frac{2d}{g}}$$

Solving for time, substituting d=0.5m:

$$t=\sqrt{\frac{2\cdot0.5\ m}{9.8\ m/s^2}}=0.32\ s$$

It will take 0.32 seconds for the object to fall 0.5 meters.