The cabin air filter cleans the air that comes through your heater and air-conditioning vents. When it clogs, the air inside the car gets worse.
About once a year, or every 15,000 to 25,000 miles (24,000 to 40,000 km). Replace sooner in dusty or polluted areas, or if you suffer from allergies.
On most cars it sits behind the glove box, and many can be swapped in minutes without tools. Your manual shows where.
No. It only affects the air inside the car, though weak airflow can make windows slower to demist.
General guidance, not professional advice. Intervals are typical ranges and vary by car, so check your owner’s manual and ask a qualified mechanic if you’re unsure.