But your MAXIMUM activity level increases by 14 every two weeks more or less, because 14 days is what do you need to get another activity time.
The formula for activity is:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
Activity is updated every 30 minutes.
Exactly.
The "time" in the formula can at most be increased by 1 every 2 week, but your activity depends on it and your post count.
If you have both your "time" and "posts" at 10 (made a total 10 posts, one in each period), you can make 130 posts right away to get 130 more activity.
The saying that you can at most get 14 activity is true only if your activity is capped by "time" but not "posts".