your question can be simply replied that  u get 14 activity  every 14 days    1 activity = 1 post 

so simply    if u post 14  or 1000000 post  in 14 days  u will get same activity  which is 14    and u will need to wait for the next 14 days to be able to claim another 14 activity

those 14 days dont start from your sign up day  they are fixed and u can check when the next 14 days will come  in this spreed sheet  https://docs.google.com/spreadsheets/d/12saLhlUoqIdairxzuSPu6EYGrt7FN2lOstO1yDjCEbA



and if u want a more detailed topic about ranking up and activity  check this great topic
https://bitcointalk.org/index.php?topic=2766177.0