i'll try to shed some light on this m64. mostly in the bible it does use real days. there are some things like dain said, but for the most part the numbers are literal.
but, i know that the theory for non literal 7 days comes from the passage that says: to god one day is as a thousand years, and a thousand years is as one day
i agree that we weren't there when the earth was created, and so it could have been made any way god wanted. i'm literally inclined, and here are a few reasons that i think might support it. but i wouldn't be surprised if i'm dead wrong, it's just this is the way i think now. on this side of fate.
one reason: since i don't believe the world evolved, god doesn't need a long time. (but he could have just as easily done it in a long time, so this isn't proof)
two reason: i think the use of the "one day is as a thousand" passage is simply meaning god doesn't exist in time like we do. he's god, he has no limitations. not even time. however, in context of creation, the first thing he makes is night and day, thus defining the "time" (days) that he has given us.
three reason: the fact that he worked six days and rested on the seventh set the foundation for our week. and the week has never been broken to my knowledge even though there have been attempts to change it. during the french revolution, the leaders wanted to change to a 10 day week, so there would be less days off work. now, i'm not ignoring the cultures who have no record of time at all. there are plenty of tribes who don't keep record of years, months weeks or days. and those people don't know how old they are exactly. they have a sketchy reconing of time, but nothing certain. but all civilized countries, that i know of, (and i could be wrong) use a seven day week. this also is no proof of a literal interpretation, but in my mind it sort of reinforces it. i hope this helped.
-mm