In simpler terms, maybe...

Group all of your posts in two week blocks
Count up those groups
Multiply that number by 14
If your post count is larger than that number it becomes your activity.
If the number is larger than your post count, post count becomes your activity.


However,  in general you should just ignore your activity because the only time it matters is to make you standout from a lookalike username that may be scamming, which should happen naturally or to meet a quota for signature spamming (coincidentally the "confused" users here are participating in signature campaigns)