Then I leaned something new. I thought that if you don't post in the 2 weeks time span you loose the activity points for that period, but looks like they are accumulated with the posts even if you do not post for a longer period of time.
That's not true... Otherwise every newbie who registered back in 2011/12/13/14/15 would be a potential hero member now...
They are only accumulated as potential activity if you at least posted once in that two week period of time.
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
Here's more or less the formula.