Much depends on the amount of use the system gets. As a general guide, the HVAC filter should be replaced every 3 months. In some homes, it is a good idea to replace it even more often than that. Large family households and homes with pets are more likely to produce extra dirt and dust so changing the filter as often as once a month would be beneficial. You might also want to consider changing it that often in homes where a family member suffers with allergies or if there are people smoking indoors.
The area surrounding the home also plays a part in determining how frequently to change the filter. If you are surrounded by agriculture, if there is construction going on in the vicinity or even if wildfires have been in the area, these are all reasons to change the filter more often.
In some cases, you will need to change the filter immediately. You should do this if it is damaged in any way or becomes damp or wet. Wet or damaged filters could allow harmful toxins such as mold spores to enter and circulate around the home.
Its clear from this short overview that in order to maintain a healthy indoor air quality, HVAC filter should be changed regularly.