Activity is calculated in this way. Post by the admin:
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)
Rough summary of what this means: If you post once per day on average, then you will eventually get the maximum number of activity points that you can get. Posting more than once per day on average is useless for increasing activity. The maximum number of activity points you can get is 1 per day, but your activity score only updates every two weeks. Therefore, it will take about 100 days to get 100 activity if you post once per day on average.
Activity is updated every hour.
See
https://bitcointalk.org/index.php?topic=582736.0 for activity update timings till 2060.
Few upcoming timings:
December 9th 2014 10:13:20 AM
December 23rd 2014 10:20:00 AM
January 6th 2015 10:26:40 AM
January 20th 2015 10:33:20 AM
See the pattern and you can find the activity timings for yourself or use, 'period number * 1210000 = a unix time' , convert unix time into your timezone.
~~MZ~~