Actually, the answer I would recommend is "till it's done."
Buy a probe thermomemter, stick the tip so it rests in the thickest part of the breast, and remove the chicken when it gets to 165F. Contrary to what one might expect, lower temperatures cause more moisture loss due to longer cooking times. I cook de-boned breasts at 500F, bone in at 425, and whole chickens at 350. In general, the fastest cooking method results in the moistest meat, because there is less time for water to evaporate. Of course, higher temperatures also make it easier to overcook something into sawdust, which is why they often get a bad rap. However, a properly positioned probe will beep the instant the meat arrives at the correct doneness. Plus, higher heat develops that delicious brown skin.
That said, 425F for an hour sounds like a recipe for cardboard chicken! Good for you for not trusting the recipe blindly.
Walt