That depends on your conditions. I always say don’t fix it if it isn’t broken. Normally I would say the filter should be changed every two to three months. The exception is if you are renovating or have pets and carpets etc. Sometimes the filter needs cleaning in less than a month if conditions are poor. In other situations the home is very clean and dust free and the filter lasts 6 months. The key is to check it monthly and replace or clean it as necessary. A dirty filter will shorten the life of your furnace and cost you money and possibly your health.