I'm not sure how long it would take, exactly.
But humans have been "out of Africa" for less than half-a-million years, and look at how Humans in general have already lightened.
So now, we have to throw in environmental factors such as UV exposure. Since higher levels of UV exposure cause darker skin pigmentation as a protective mechanism. So if people with lighter skin go back to Africa (say), their offspring will tend towards darker skin.
That's why I say whites claiming they are "the master race" are a hoot. It's just so ridiculously ignorant.