How many feet do you free fall per second?
Then, how far do you fall in 6 seconds?
According to the rules, a creature falls 100 squares (500 feet) in the first (six second) round. According to the laws of physics, a creature falls y = 0.5 g t2 feet, where g is the acceleration due to gravity (32 ft/s2 on Earth) and t is the number of seconds in free fall.
Considering this, how far will a person fall in 3 seconds?
Acceleration due to gravity is 9.8m/s per second. During the next second you are already falling at 9.8m/s and accelerating to 19.6m/s. So you fall another 9.8 meters, plus 4.9 meters to account for the acceleration. During the third second, you fall 19.6 meters plus 4.9 meters to accout yet again for acceleration.
On average, it takes one second to fall 200 feet. That said, it does take a bit of time to accelerate up to what's called your 'terminal velocity'.