The honest answer: it depends. Some homes go five or seven years without trouble; others need attention sooner. Pets, allergies, recent construction, and how often you run the system all matter.
A reasonable rule of thumb
If you have pets, allergies, or a smoker in the home — every two to three years. Otherwise, every three to five is usually plenty. The best signal is your own eyes: pull off a return-vent cover and take a look.
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