Typically in central Texas, a 30 day interval is recommended between filter replacements. During early fall and early spring, when operation is sporadic due to our unpredictable weather, you may be able to extend this interval to 45 or even 60 days.
Filters are cheap and normally easy to replace.
It includes higher operating cost, poor cooling, condensation leaks, frequent equipment failure, and shortened equipment life. Healthwise it means a decrease in the air quality you and your family breath due to the presence of more dirt and allergens, increased mold and algae growth, and rust. Most systems should be checked twice a year. In the spring for cooling and in the fall for heating. Independent studies prove the investment in preventive maintenance is pays for itself through lower operating costs and fewer expensive breakdowns.