It doesn't continue to increase from the day you register, you must be active on the forums. If you skip (don't login/post) a 2 week activity reset period then there is no way to get it back and I believe that's what happened to you.
There is no specific time, it keep increasing each two weeks (from what I can see, Its like 6-7mn increase): https://docs.google.com/spreadsheets/d/12saLhlUoqIdairxzuSPu6EYGrt7FN2lOstO1yDjCEbA/edit#gid=1012758442