We change them about every 3 months, more often after we drive on dirt roads or in very dirty conditions.

By the way, it is healthier to bring outside air into your car through the cabin air filter than to continuously recirculate the small amount of air in your car. All the artificial surfaces "outgas", people and pets breathe out carbon dioxide (and germs) - these pollute the air in the car - exchanging with fresh air dilutes this pollutants. Think of this as being safer to gather outdoors with others - larger volume of air there is a lower concentration of "bad stuff" - the exception being when you are driving near smoke. Allowing in fresh air also reduces moisture, so your windows don't fog up.