So this is an instance where d&d and real world don't mesh
Basically Terminal velocity in the d&d universe is after 200ft. The rule being: 1d6 points of damage per 10 feet fallen, to a maximum of 20d6. (DMG)
Terminal velocity in the real world is ~122mph (converted to feet thats 179ft/sec) and it takes 500ft to reach it, however, that is a falling body with a random orientation. Speeds of up to 600mph have been reached from skydivers orienting themselves into a low wind resistance shape.
Accelleration increases asymptotically up to terminal velocity. The above numbers of 122mph and 500ft are for real world wind resistance at the surface of the earth.
to figure out how far somone falls from rest in a certain amount of time, you have to add things together,
approximating g as 32ft/sec^2 a person will gain 32feet of speed each second,
after one second his velocity is 32feet/sec and he has traveled 32feet.
After two seconds his velocity is 64 feet/sec and he has traveled a total distance of 96 feet. After 3 seconds his velocity is 96 feet/sec and he has traveled a total distance of 192 feet.
4 sec, velocity is 128ft/sec, distance traveled is 320.
5 sec , v=160, dt= 480
at somwhere between these times terminal velocity will be reached. (around 179 ft/sec-(122mph converted to feet/60/60)) (this math also supports mythbusters btw)
6 sec, v=178, dt = 658
and you can extrapolate from there. The point is, in d&d terms you will reach "terminal velocity at half a round", whereas in real life it takes a little more than 5 secs to reach terminal velocity.
remember, air density and gravitational factors may also play a part in accelleration, but for our purposes the above numbers are close.