Depends on what you have. Most newer house only have one furnace filter, you only need to change it every 3 month. Some newer thermostat actually keep track how many hours it runs and suggest you change the filter when a total of hours is reached. Some older house have a filter at each 出风口 or return air vent. You should replace them when they get dirty. (since the amount of filter surface area are increased, it should take longer to get dirty)