Billiard Ball Model and Momentum Post at DUHere.
Here's a comment I made that I think sums up a key point:
If the top of the south tower or north tower truly broke off as a chunk and tipped, one could imagine that this large weight would overwhelm the floor slabs on one side directly underneath and fall very rapidly with little slowing. But in that case, one would expect to see something very different than what was seen:
a) more of an asymmetric collapse with the lower core and floors on the opposite side of the tipping side left relatively intact
b) this upper chunk start to progressively fall away from the lower floors and eventually completely fall away from the rest of the tower, leaving the lower portion of the tower intact
If the lower core below the collapse initiation needs to be taken out (as what happened on 9/11), that is going to offer huge resistance and slow things down much more.
So really, the timing very much depends on how the collapse proceeds. If it is complete symmetric collapse involving the core, it will be much slower than what was observed on 9/11.