As a guideline, most manufacturers recommend replacing your furnace and air conditioner filters every 6 months. However, what you really need to do is check the filters every 6 months. If you see that dust and debris is not accumulating very quickly, you may be able to go more than 6 months on a filter. But if you see that the filter is getting very dirty very fast (perhaps because you have a lot of pets or dust in the home) you may need to replace it more often than every 6 months. The main goal is to avoid running the system on a dirty filter, as a filter that is full of debris presents more resistance to airflow and therefore reduces your HVAC efficiency.