Every 2 weeks your activity will rise by 14 AS LONG As you post at least once every 14 days
The activity level is also capped by post count, so your activity may not go up by 14 each period.
If you make 5 posts in 14 days, and make another 5 posts in the next 2 weeks, your activity would be 10 instead of 28.
But your statement is true for all active people (more than a post a day on average).