Apparently , the activity changes every 1210000 units of the UNIX timestamp. This equates to 14 days , 6 minutes , 40 seconds.
Here is a list of all the ACCURATE start dates of the 14 day periods till 2060 !
https://docs.google.com/spreadsheets/d/12saLhlUoqIdairxzuSPu6EYGrt7FN2lOstO1yDjCEbA/edit?pli=1

It used to be 30 mins or that's what was stated, but the sticky has been updated to 60 mins now which would seem right: https://bitcointalk.org/index.php?topic=178608.0