The only real answer to this is to change your furnace air filter when it needs to be changed, this can vary dramatically based on several things like, how thick the filter is and how much particulate it is capturing. Generally, 1” filters last between 1-3 months but a 4” filter can last 6-12 months. It may be necessary to replace the filter more or less often depending on your home.

Having pets in the home will generally reduce filter life. Using cool mist humidifiers or essential oils with tap water will greatly reduce filter life because the minerals in the water will find their way onto the air filter. Larger households with more powerful furnaces may have to replace their furnace air filter more often, potentially each month to retain maximum benefits.

One good way to see if your furnace filter is clean or dirty is to hold it up to bright light and see how much light comes through the filter. Comparing that to a new filter will quickly give you an idea of how dirty the old filter is. Once you know how long your filter generally lasts you can plan on a replacement schedule. Since filters are inexpensive you may choose to just replace it before it needs to be replaced.