What would be the minimum activity, 2000?
For Jr. Member it's 30, for Member 60, for Full Member 120, for Sr. Member it's 240, for Hero 480 and for Legendary (although random) it's around 960.
Then the logic answer would be 1920, either exactly that or somewhat random around that. So about 5 years and 3 months of full activity would be required.
If you do it I would hope they keep the randomness of legendary. So have a range you get it within but it is a big range.
Like how I'm still hero but could hit legendary any activity change. Makes it kinda fun. So please keep the random feature if you do add another rank above legendary