fractions of a second
IF (A big "IF") that is true, I suppose that for one reason or another, the charges on each floor may have triggered accidentally as the upper floors crashed into each descending floor. That would account for what some conspiracy folks say about the charges being in evidence as the buildings collapsed.
The only reason I say that is because Building 7 could not have been wired in just a couple of hours before it was demolished. Building 7 must have been wired long before 9/11. If it was, then that is reason to consider that the Twin Towers were also pre-wired.
Building 7, BTW, came down from the bottom, all in one piece. A pre-announced demolition, according to several reports. All three networks video-taped the demolition, but it wasn't generally reported as a demolition for some reason.
I'm just trying to piece together a scenario from bits of info I have read during the last few years, not that I necessarily believe it.