A: You should check it at least every 30 days. While the manufacturer may recommend changing it every 90 days, the simple truth is you change it when it is dirty. As dry as the weather is here some filters may not last two weeks. Add pets and high traffic to some homes and you would be surprised how quickly filters get dirty. Keeping filters changed regularly throughout the year is important for good air flow and is one of those minor maintenance efforts that can prolong the life of your system and save you money in costly repairs. Pleated filters are better than flat filters.