Most footage of the WTC collapses was taken from a distance, for obvious reasons, and the clouds of dust make it difficult to determine the precise collapse times. But there are videos taken from closer to the disaster which can at least point us in the right direction.
Take this view of the first collapse, for instance (click the image for the video, 4.4MB in size).
The rumble of the collapse appears to be noticeable as the video starts, or at least very close to it. If the sound is originating at a height of 965 feet (roughly the 78th floor), perhaps 200 feet from the microphone, then that’s a distance of around 985 feet, which means the sound began 0.86 seconds before we heard it (taking the speed of sound at 25 degrees C of 1136.2 feet per second). We’d suggest it’s realistic to assume the collapse started at 0.5 seconds before the clip begins, and perhaps a little earlier.
The cameraman bravely films what’s happening for a few seconds, before realising this may be a good time to run. We then lose sight of the building until roughly the 11 to almost 12 second point, where a considerable number of storeys of the building are clearly still standing. 25? It could be more as falling debris obscures the upper levels, but what’s clear is that the collapse didn’t end until some time after the 12 second point, putting the overall collapse time from this video as “more than 12.5 seconds”.