Yes, you'll need to post now and you rank will raise rapidly to some level (+1 for each post), but later when it will reach maximum you'll need to wait as all other users for 2 weeks. Next update of the ranks will be approximately in 1 week. Maximum you can get 14 points for 2 weeks (1 activity point for 1 day)
Does it mean if I have a high posts, then when the time for upgrade comes, I will go up to Jr. Member or full members as the case maybe or I will still move in gradually as every other member will?
It takes 14 activity for every 2 weeks period no matter how much you post, you need time. Check this:
https://bitcointalk.org/index.php?topic=237597.0If you want to become a Jr. member, it needs 30, for Full member it needs 120 and so on, so your number of posts doesn't make any sense but what makes sense is that if you make just one post during that 2 weeks time, then you are eligible for the remaining 13 activity and you can get it by making 13 posts in a day or two. Additionally, this can be done for each 14 days cycle, so let's assume that you made one post every 2 weeks for 6 weeks in total (3 x 2 weeks) then you are eligible for 42 activity and you can go Jr. Member in a day if you manage to do 39 posts in a day (just kidding, don't do that as it will be considered as spam) or in a few days.