When poaching chicken using my method (cold water brought to a low simmer) then the amount of time it will take depends on a few factors. The volume of water used will have a significant impact since you need to bring this to a low simmer after adding the chicken. The more water you use, the longer this will take.
Once the water reaches a low simmer it will take around 10-12 minutes to poach a medium-sized chicken breast (around 120-150g / 4-5oz). If you are using larger breasts you may need to increase the cooking time by an additional 5-10 minutes. The best way to check that it is done is to use a kitchen thermometer. Remove the chicken when it reaches 68-70C (155 - 158F) then leave it to rest for 5 minutes which will allow it to come up to the recommended temperature of 75C (165F).