The shape of a cast charge will, to some extent, modify the length of time that it will burn. Juggling the proportion of calcium sulphate to aluminum can change the rate as well, but there is a poiojnt at which it will not sustain itself.
BTW, any thermite used in demolitions of this scale would have to be a cast charge composed of ferrous oxide, aluminum and calcium sulphate with some sort of refactory coating or shell. They would have to be numerous just to do a job on the scale seen, and they would have to be enormous to be able to retain any heat all the way to the street level.
Now try hiding a couple dozen Buicks inside the walls on an occupied floor of an office building.