You should replace a 16x25x1 furnace filter every 1-3 months. This timing can change depending on indoor air quality, how many people live in your home, pets, and the filter type. If you have allergies or pets, you might need to replace it more often, like every 1-2 months. Check the filter regularly and replace it when it looks dirty or clogged to keep your air clean and your furnace running well.