The activity number is determined in this way:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
i don't quite understand what time is in terms of 2 week periods. Just to clarify, if i joined on jan 1 and it is feb 1 today, does that mean time=2?
Here is the spreadsheet where you can find the dates of the change of the activity, it was mentioned in a previous post here