Free-falling from WTC heights
The towers were 1350 and 1360 feet tall. So let's start by using our trusty free-fall equation to see how long it should take an object to free-fall from the towers' former height.
Distance = 1/2 x Gravity x Time(squared)
or
Time(squared) = (2 x Distance) / Gravity
Time(squared) = 2710 / 32 = 84.7
Time = 9.2
So our equation tells us that it will take 9.2 seconds to free-fall to the ground from the towers' former height.
Using our simpler equation, V = GT, we can see that at 9.2 seconds, in order to reach the ground in 9.2 seconds, the free-falling object's velocity must be about 295 ft/sec, which is just over 200 mph.
But that can only occur in a vacuum.
Since the WTC was at sea level, in Earth's atmosphere, you might be able to imagine how much air resistance that represents. (Think about putting your arm out the window of a car moving half that fast!) Most free-falling objects would reach their terminal velocity long before they reached 200 mph. For example, the commonly-accepted terminal velocity of a free-falling human is around 120 mph. The terminal velocity of a free-falling cat is around 60 mph. (source)
Therefore, air resistance alone will make it take longer than 10 seconds for gravity to pull an object to the ground from the towers' former height.
Observations from 9/11
On page 305 of the 9/11 Commission Report, we are told, in the government's "complete and final report" of 9/11, that the South Tower collapsed in 10 seconds. Here is the exact quote: "At 9:58:59, the South Tower collapsed in ten seconds". (That's the government's official number. Videos confirm that it fell unnaturally, if not precisely that, fast. See for yourself: QT Real)
But as we've just determined, that's free-fall time. That's close to the free-fall time in a vacuum, and an exceptionally rapid free-fall time through air.
http://911blimp.net/prf_FreeFallPhysics.shtml