This is more of a method than a recipe. Just adjust it according to the amount of chicken you wish to poach and the size of your fillet.
I always buy free range chicken breasts, which are typically larger than those produced from caged animals, so the timings are based on these larger breasts.
If your chicken breast is not completely cooked after the 15 minutes resting time, return the chicken to the water and allow to sit for a further 5 minutes. If you feel the water temperature has dropped too much to continue cooking the breast, just reheat the water slightly first.