An extremely long time. IRNs are notoriously difficult to tame, and they don't follow the rules of most birds for taming. If you do get one, here are some tips.
Don't clip the wings. I can go more into it later if you want, but clipping the wings of a wild IRN does a lot to sabotage trust efforts and heighten paranoia in your bird.
Take your fingers out of the equation. Whenever possible use a fist and hide your fingers until your bird is "tame to your fist" and trusts it, then slowly introduce fingers. It's a weird IRN finger phobia thing.
But seriously, if you want a tame IRN, I highly suggest buying one that already is. Most people are NOT up for the challenge of taming one, and many will never become tame no matter what you do, especially females.