How quickly did the towers fall, exactly? You’ll find different answers depending on where you look.

Around 8 seconds

The Twin Towers collapsed in about 8 & 10 seconds respectively
http://www.checktheevidence.com/911/TwinTowersandGravity.htm

8.4 - 12 seconds

The videos and seismic records show that the time of one structure's destruction was approximately 8.4 seconds though the complete settling of the building lasted slightly longer, perhaps as long as 12 seconds, but not long enough to account for anything but explosives.
http://reopen911.org/Contest.htm

<= 10 seconds

This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of 200 km per hour. If it had been free fall, with no restraint, the collapse would have only taken eight seconds and would have impacted at 300 km/h
http://www.tms.org/pubs/journals/JOM/0112/Eagar/Eagar-0112.html

<= 10 seconds

Each WTC building collapse occurred at virtually free-fall speed (approximately 10 seconds or less).
http://www.lewrockwell.com/reynolds/reynolds12.html

10 seconds

At 9:58:59, the South Tower collapsed in ten seconds...
Page 305, the 9/11 Commission Report

10 - <14 seconds

According to testimony provided to the 9-11 Commission, the tower fell in 10 seconds. Other data shows it took closer to 14 seconds. So the towers fell within 0.8-4.8 seconds of freefall in a vacuum. Just like WTC7, this speed seemed impossible if each of the 110 floors had to fail individually.
http://www.physics911.net/closerlook.htm

14 - 16 seconds

Each of the Twin Towers totally collapsed in an interval of approximately 14 to 16 seconds.
http://911review.com/errors/wtc/times.html

15 seconds

...video evidence of the North Tower collapse suggests that it took close to 15 seconds for the destruction to reach the ground.
http://911research.wtc7.net/wtc/analysis/collapses/freefall.html

15 seconds

Each of the Twin Towers totally collapsed in an interval of approximately 14 to 16 seconds.
http://911review.com/errors/wtc/times.html

15 seconds

Both demolitions took place in about 15 seconds, which is about the time it would take for a free-fall from that height.
http://911review.org/Wiki/TwinTowers.shtml