If you use frozen chicken, I would do 20 min HP to be safe (although 15 min has worked for me in the past. I’ve found it depends on whether I laid my chicken flat to freeze.) Note that if your chicken is frozen in a big ball, it may not cook thoroughly in the center. If you find your chicken is not cooked through, my quick trick is break it apart and set pot to Sauté. Chicken will finish cooking in the boiling broth.