Pretty sure they're all two weeks, except the first one.
From the OP:
''Classical Age's feature period is one week long (from 12/12 - 12/19) while all other Ages are two week periods, sharing time with the Age before and the Age after''
Disregard post. I was confused with the wording for when instant upgrade happens. It makes more sense when you click on the link for the ages where the rush has started