This question assumes that no method of controlling speed or direction is available, and that the falling is taking place from a location 30' higher than necessary in order for said creature or object to reach the maximum falling damage of 20d6. The fall occurs from said height, proceeds directly, and impacts the ground of solid rock at the resulting velocity.
Thus, what is the movement rate of said falling creature or object each round?
Flying creatures that fail to maintain minimum forward speed fall 150 feet the first turn, and 300 feet each turn thereafter. Those creatures presumably have wings or whatever and are therefore able to produce significant drag/lift that limits their falling speed, however—a human being falls a lot further than that in 12 seconds in real life (though interestingly 12 seconds is roughly how long it takes to reach terminal velocity).
Manual of the Planes uses the same numbers for subjective gravity, which kind of makes sense when you consider that a being is capable of a certain amount of control over gravity in an area with subjective gravity.
Dragon vol. 327 has a Sage Advice column that considers the question. Andy Collins does some back-of-a-napkin math using Earth’s gravitational acceleration (\$32 \frac{\mathrm{ft}}{\mathrm s^2}\$), and comes up with 576 ft in the first round, which he suggests you round to 500 feet for simplicity.
After that, we start having to be concerned about terminal velocity, and sure enough, this is where the Sage brings that up,
If the Sage remembers his high school physics, terminal velocity for a human body is roughly 120 mph (equivalent to a speed of 1,200 feet per round, or 200 feet per second); thus the character’s falling speed hits its max in the first second of the second round.
This is kind of problematic. The 120 mph figure isn’t too bad, but the 1,200 feet per round figure clearly stems from the extremely-rough estimation of 120 mph as 200 feet per second—it’s more like 175, and multiplied by 6, that adds up. The actual number is a bit over 1,000 feet per round, rather than 1,200.
Then again, a web search finds sources claiming human terminal velocity is anywhere between 120 mph to 150 mph. All the figures I can find are assuming the human is laying flat, spread-eagle, to maximize drag. So actually achieving 1,200 feet per round is probably plausible, and it may even be possible to substantially exceed it—one skydiving site claims humans intentionally diving to maximize their speed can achieve velocities exceeding 200 mph—that gets you 1,760 feet in a round (which my high school education tells me is one-third of a mile, since 1,760 is the number of yards in a mile and there are three feet to a yard—aren’t American measurements fun?).
Then we get this... oddity:
It’s safe to say that after two rounds the character will have fallen nearly 2,000 feet, and will fall another 1,200 feet per round thereafter.
The problem with this is that to reach 2,000 feet after the second round, when the first round had 500 feet, means the second round involved a fall of 1,500 feet—greater than the “terminal velocity” of 1,200 feet used by the Sage.
For my own preference, I would use 500 feet on the first round and 1,000 feet thereafter—simple, clean, easy to remember. If a player was really adamant about getting down there now, I’d probably let them push the second-round-and-later speeds up to 1,760 feet if they want. If I really cared I’d probably also assign a falling damage of 35d6 rather than 20d6, though, since you’re traveling a good deal faster than the “usual” terminal velocity that informs the 20d6 limit.
The generic answer is "none", or "infinite".
Falling is defined in several handbooks, including the Rules Compendium, and they make no mention of falling speed or falling time whatsoever. You can create such notions, and they could be useful sometimes, but that would be a house-rule, because it would be something that you made up as it isn't in the rules.
Therefore we can deduce that falling takes no time, it's instantaneous, and there is no falling speed, just falling damage which depends on how much distance you fell. Even if this goes against common sense.
Think of ranged attacks, this same reasoning can be applied to arrows, nowhere in the handbooks are rules for calculating projectile speed and time to hit the target, it takes the arrow the same amount of time (no time at all) to hit a target adjacent to you or 1000 feet away, yet that's not what would happen in reality, same thing is happening here.
The game system just doesn't contemplate falling as something that lasts any amount of measurable time, falling is resolved like an action in the same turn it happens, the same way you resolve an attack or a move. Certain things could happen during a fall, like attacks of opportunity, or immediate or readied actions, but that's about it.
You can think about it being like an involuntary free action.
Some spells or abilities that allow you to fly do specify what happens when you fall while using them, but that only applies in those situations.
This is usually fine in most games, but there are instances where it isn't, but what happens in those cases is mostly up to the DM, because each creature has different air resistance, each planet different gravity and atmosphere, and each plane different physical laws.
