One question, since my account is quite old (December 2014) but I was mostly lurking around (Offline) my activity will rise only for the time I am active right?
Activity periods are 2 weeks unix time, a full list
here. If you post at least once in one in the two weeks period, you would gain a potential 14 activity. To make them appear, you have to make the amount of post that is equal to the potential activity. Activity will update every 30 minutes or so.
That is great share to know about activity points.