At that temperature, the growth of harmful bacteria will be halted — indeed, foods that are kept constantly frozen at 0°F or lower will keep safe indefinitely, notes the FDA. (When frozen food begins to thaw, however, bacterial growth will resume.)
The best way to ensure that your freezer is sufficiently cold is to use a freezer thermometer. You can find inexpensive models at most grocery and hardware stores.
Bear in mind, too, that although foods may remain safe while they’re frozen, they will eventually start to go downhill when it comes to texture, flavor and taste. To ensure that you enjoy the best quality, consult the Keep It or Toss It database for optimal freezer storage times.