From my experience, the timing of irimi, kokyo, and most other techniques depends up on your partner. The speed and ferosity of the attack will dictate your response. i.e. You will meet the attack and blend. As far as I know, the only way to learn this is by going to class often and working with as many different partners as possible, because every person you work with will give you a variable amount of speed and energy. Hope this helps with your question.