Filter replacement frequency depends on several factors:
Standard recommendation:
1-inch disposable filters: Every 30-60 days
4-inch pleated filters: Every 6-12 months
Washable filters: Clean monthly, replace every 5-7 years
Replace more frequently if:
You have pets (every 30 days)
Anyone has allergies or asthma (every 30 days)
You're running a renovation project (weekly during construction)
You live in a dusty area
You run your system constantly
Check monthly regardless of schedule. If filter looks dark or clogged, replace it even if the timeframe hasn't elapsed.