One inch furnace filter should be changed after one month, 4 inches ventilation filter after 3 months. Certain conditions in your home will significantly add particles to the air. this will cause the filter to capture more particles than usual, shortening its life to less then 3 months or reducing the effectiveness of the ventilation filter to capture particles. these conditios include:

• Dirty ductwork
• Pets
• Construction Work
• Burning Candles
• Tobacco Smoke
• Fireplace and Wood-burning stove Smoke

Also, running your furnace or air conditioning fan continuously will help clean the air in your home but could further shorten the life of the filter.