Be sure to melt the butter and mix it with the herbs first. This will allow time for the butter to soak up some of the lemon flavor from the thyme while you are getting the chicken ready.
The roasting time will vary depending on the size of the chicken breasts. If the chicken breasts are very large, use a roasting pan with a rack to allow the chicken to sit above the bottom of the pan. Since the roasting time will be longer, the butter and any drippings could begin to smoke, so add about a cup of water to the bottom of the pan before placing in the oven.