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Take a look at this thread too (besides those referenced above):
https://bitcointalksearch.org/topic/activity-new-membergroup-limits-237597You seem to be confusing time ("number of two-week periods in which you've posted since your registration") with login time on this thread. Login time has nothing to do with Activity. Activity is based merely on posting. What the formula indicates is that, no matter how many posts you create, you will get a maximum of 14 Activity points every 2 weeks. You can get less obviously, just not above 14 every fortnight.
The rough idea is that you need to post at least 14 posts every 14 days to max out (it’s a little be more permissive, as the formula allows you to accumulate Activity from excedent posts belonging to previous weeks, providing you make at least one post in the activity window).
Say you start now from scratch, and during a two week period (*) you comply with one of the following four distinct sample scenarios:
a) make 0 posts -> 0 Activity will be generated
b) make 1 posts -> 1 Activity will be generated
c) make 15 posts -> 14 Activity will be generated (the maximum in a two week period)
d) make 155 posts -> 14 Activity will be generated (the maximum in a two week period)
Say that during the following (two week) period you make 1 single post. Your activity will now increase (continuing with the above scenarios):
a) +1 Activty -> totalling 1 Activity
b) +1 Activty -> totalling 2 Activity
c) +2 Activty -> (1 post that rolls over from last week + this week's new post) totalling 16 Activity
d) +14 Activty -> (13 post that rolls over from last week + this week's new post) totalling 28 Activity
(*) Activity window period really, according to a fixed calendar.Minimum 1 post per activity window required to increase activity.
You should barely be concerned with activity nowadays. If you are concerned with ranks at all, it’s the Merit requirement that is going to prove hardest to achieve (activity is a piece of cake in comparison).