When your air filters seem dirty from a visual point, you need to change them. However, there a number factors that an owner can be used as a standard, depending on:
1. Type of filter your A/C system requires, the overall air quality of your city where your home is located.2. Pets – Birds, cats, dogs, hamsters (do you have one?), etc.
3. Number of occupants in the house
In summary; Home that is not occupied by regular base or single occupant homes without pets or allergies may get away with changing the filters every 6-12 months, but typical suburban home with that is used 90% daily and pets will probably need to change every 90 days.