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
Is it really takes so long? If I made post 20 weekly , it takes 50 weeks. also I just counted 1 merit per post . There's a possibility to get a number's of merit in one post if it's qualityful also there`s no garuntee that I can get merit in every post. So I just calculate it in averaged.
I belive it's possible to earn legendary member ship in 1 and half year if some one try hard to achive it . Althouth it's matter of quality not quantity. If we focus on our quality then it's possible .