The time frame your furnace filters are supposed to last can vary drastically. Checking them frequently will help homeowners know how often they need to replace them. While people may find some filters last up to three months, they need to realize this only applies with normal usage and in homes without pets. Numerous factors affect filter life such as home size, home location, how much you run your furnace fan, how many people (and pets) live in the home, filter size, and filter type. The more efficient the filter, the more dirt it will collect, and the faster it plug up. But this is actually a good thing- this dirt would otherwise be in your home and your lungs. Purchasing good quality filters and changing them regularly is an investment in your health. It is recommended to check your filters monthly and change them if they look dirty regardless of how long they have been in use.