You are guaranteed to become Legendary somewhere between 775 and 1030 activity
LOL
I'm less than a month from becoming heroic and you introduce Legendary status which is another year away for me.
By the time I'm legendary will there be "Immortal" status to make us all wait another year?
LOL
What?
Man, I love it...immortal is the next level but make it impossible to get, like activity 10,000 and above. lol.
So far they've been going up by factors of 2. I think that legendary is basically 960 activity +/- some amount and then chosen randomly. Still the center is 960 which is 2x 480 (hero) which is 2x 240 (senior) and so on.