I think there is little wrong with assuming that the probability of appearance of life given right condition and time to develop is one
Fair enough. It's a bit like the infinite monkeys in infinite typewriters scenario. Given enough time, sure, you'll have life. It's a question of how much time. Still, assuming all Eart-like planets will actually develop life given the time lapse of the existence of the Milky Way does still seems like a bold assumption.
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).
Fair enough. It's a bit like the infinite monkeys in infinite typewriters scenario. Given enough time, sure, you'll have life. It's a question of how much time. Still, assuming all Eart-like planets will actually develop life given the time lapse of the existence of the Milky Way does still seems like a bold assumption.