Jonathan - a single celled organism will never mutate into a housefly, because individual organisms do not evolve - populations do.
There's more to your misunderstanding. There is not a single father-son lineage of organisms in a population, so it's not a simple case of say 1 mutation per generation, 1 favourable per x generations. There are many organisms per generation (and of course reproduction within the population is not organised into discrete generation events). So figures are rather hard to generate. How big's the population? What is the mutation rate per million base pairs? How good is the error prevention mechanism? How big is the genome? What proportion of it is functional?
And so on.
Evolutionary change rates are measured in Darwins, and a very accessible article on research into the subject is to be found here:
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