When your air filters seem dirty from a visual point, you need to change them. However, there a number factors that a owner can 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 occupy on 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 are used 90% daily and pets will probably need to change every 90 days.