First, you need to reach ~1000 activity. Max, you can earn is 14 activity on bi-weekly (you should need to post about 14 posts in two weeks to get that).
So, 1000/14 = 71.42 periods are required to reach the activity limit for Legendary member status, which is somewhere around less than 3 years.
Plus you need merit (but probably you already know that). In case you want to be a Legendary member, you have 3 years to collect 1000 merit