The trick is to cook them through so you don't have risk of salmonella or other bacteria, but don't overcook to keep them moist and juicy for flavor. My Italian grandmothers from the old country used to cook chicken breasts for hours because they grew up in the country without refrigeration, but the result tasted like shoe leather!
25-30 minutes for boneless chicken breasts should do it, but very large bone-in chicken breasts can take up to 45 minutes to cook through. If you don't have a meat thermometer, you can tell by using a cooking fork to pierce on through all the way down to right next to the bone. If the juices that come out are clear, it is done, if pink, it is still raw inside and needs a little more time. Be sure they are thawed completely before you put them in the oven, or they will come out leathery on the outside but still uncooked at the bone.