Well, that is, my 1st first post in a new 14-day period always changes the amount of my activity, the last time it was on June 30th, the next it will be on July 14th, then July 28th. It turns out that I can only count on a new rank these days?
Yeah the first post in the 14 day period adds 14 activity to you.
And yeah you'll just have to wait for the next periods to be able to rank up.
There's no predetermined activity points to get legendary though and I think it was set to be a random number and people have waited a long time to get it in the past (some right at the end of the 1100 limit). I just wouldn't wait on it, I think merit is the better stat anyway.