The answer
Inspect 1-3" filters monthly; replace every 30-90 days (every 30 days with pets/allergies/smoke). 4-5" media filters last 6-12 months. MERV 8-11 is the ideal residential balance (dust, pollen, mold) without excessive static pressure drop. MERV 13 captures PM2.5, smoke, and virus carriers; verify system static pressure tolerance.
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.