Everything below is assuming you have the merit required and @theymos hasn't spoiled it to you.
Me and a few more have this privilege to know when they will hit the Legendary rank.
Legendary rank needs a random value from 775 to 1030 in activity status.
Assuming a maximum of 14 activity every 2 weeks we can calculate:
(Activity max-Activity min)/14=(1030-775)/14=18.2 resets.
Since we can't have a decimal value for obvious reasons we get 19 chances to hit the rank up.
100/19=5.2%
Only 5% of Legendary knows when they will rank up on the last reset
Nothing more just writing my paranoia here.