The simple answer is "when it becomes dirty and can’t do it’s job any longer." But there are other factors that need to be taken into account because they affect how long a filter can last before it becomes inefficient. There is a huge amount of technical information out there on this subject which will send you to sleep in 15 seconds flat, but don’t worry, we’ve done all the background work for you. Here is what you need to know... in as few words as possible.
The more people and pets living in your home, the more frequently your filter will need to be changed. People generate dust and dirt in the air and pets contribute their own kind of dust and pet dander. Neither is good for your indoor air quality.
The last thing you want is nasty allergens indoors repeatedly circulating through your a/c duct system. A good-quality, pleated air filter will capture microscopic allergens and it’s a good idea to change your filter more frequently. Especially if anyone in your family has respiratory issues or is an asthma sufferer.
Is outdoor pollution an issue where you live? For example, are you close to a busy highway system, landfill or construction site? Outdoor air quality affects indoor air quality. Local climate can also be a contributor to poor indoor air quality. If your system is constantly running to cope with high heat and humidity in summer (southern states) or extreme cold in the winter (northern states), your filter will need to be changed more frequently. On the other hand, if you live in a more temperate climate, you won’t need to change it as often.
Take a quick look at your filter every 30 days. If there is a coating of dirt and grime on the surface, and you can’t see the filter media, it’s time to change it out. Filters are inexpensive and compared to the cost of a larger than necessary energy bill and expensive HVAC equipment repairs, it’s more than worthwhile.