An Iron ball and a wooden ball of same radius are released from a height h in vacuum then the time taken by both of them to reach the ground

Acceleration due to the gravity of an object on the surface of a planet is given as:
$g=\frac{GM}{R^2}$

Where M is the mass of the planet, G is the gravitational constant and R is the radius of the planet.
As we can see, acceleration due to gravity does not depend on the mass of the object falling therefore, both bodies experience the same acceleration due to gravity. 
So, both will reach the ground at the same time.

In our day-to-day life, we don't experience the same because of air resistance. Lighter objects experience more drag, therefore they fall slowly as compared to heavier objects.