13 weeks seems like a fair number. People like me who are committed to Hive would, of course, love a smaller number so we could swing trade easier, but that would seemingly open the door to abuse by people not so committed to Hive—or if not abuse, just taking the money and running. So I have no problem with 13.
It might be interesting to introduce a faster power-down in exchange for 5-10%, as @bozz and @steevc suggest; I'd support that.
RE: How many weeks do you think powerdown should be?