You will achieve that in 2weeks with 3 posts per day, you need 100merits though before you can become a sn member.
No, 100 merits is needed to become Full Member, while to become Sr. Member you need to get 250 Merit. And about activity - you can get max 14 activity in two weeks period. So, it's enough to make one post per day to get max activity. Anyway, activity doesn't means much if you don't have Merits earned.