I am not a geologist, but the Wikipedia article on the Late Heavy Bombardment (http://en.wikipedia.org/wiki/Late_Heavy_Bombardment#Geologic...) says that 100 million years is the time that an Earth-sized body would take to to cool down and form a crust (the bombardment basically tuned the Earth into a ball of molten magma).
Your point is correct, but my hunch is that the time needed is no more than a million years or so (small on the galaxy scale), possibly less.