Each home is a little different. You only need to change the filter when it’s too dirty to let air through. Pets, dirt, and dust can cause you to need to replace your filter more often. If you are unsure how often to replace your filter, take it out and hold it up to the light. If you are not seeing light through it, you are not getting enough air through it.
At a minimum replace your filter at the beginning of a new season, during the spring and fall. When you replace it, if it is very dirty, try replacing it every 3 months.
The purpose of the filter is to keep dirt and debris from damaging the internal components of your cooling system while still allowing enough air to move through the system. A dirty filter causes more strain on the electrical components causing them to fail sooner and your energy bill to be higher than it needs to be.