From the vBulletin site:
Description
This hack adds an activity metric for users to your forum; this metric takes into account a variable number of weeks. Also provided is a Junior/Normal/Senior member system based on this metric, and a longevity metric (user registered time as a percent of the amount of time the first user has been registered).
Visiting each day and posting the configured amount of times per day for the configured number of weeks will yield 100% activity. Posts are checked individually for each day, so days cannot be skipped then spammed up afterwards. The weight of visiting for the day relative to posting is configurable. Weeks are weighted so the most recent has a bigger impact on the activity percent than the most distant.
So the most recent weeks are weighted higher than previous ones. As for how many weeks, only Jeremy knows.