The equation to calculate a free-falling object's velocity or time spent falling is velocity equals gravitational acceleration multiplied by time. This occurs if three conditions are given: an initial velocity of zero, a hypothetical infinite space to fall in and negligible air resistance.
Take for example an object falls for 120 seconds. Multiplying the time by the constant gravitational acceleration of the Earth, measured at 9.81 meters per second squared, the final velocity is 1,177.2 meters per second.
Factors like initial velocity and air resistance, respectively, increase and decrease the final velocity. Drag increases with the square of the final velocity and can eventually cancel out the weight of the object, creating a terminal velocity. This is the absolute fastest speed an object drops given these conditions.