my apple store said 14 days for color exchanges. i can't be bothered. the space grey isn't awful at all, plus i have a case, so its not like anyone would see the back anyways.
I think that is what apple is hoping many will do.
Can you just image the thousands of refurbs in the first week of sales of space grey they will get from the people that got it just because they told them they could exchange the color.