The standard recommendation is to replace furnace filters at least once every 90 days. This is a general guideline, however, and there are several factors that can affect the frequency of filter replacement. For 1 and 2 inch filters, the 90-day recommendation is a good starting point. Thicker filters, such as 16x25x1 models, will last longer, so if you have a thicker model, you may be able to extend the time between replacements.
If you have several pets in the house, it is advisable to change your filter more frequently, up to every 30 days. This will help keep your furnace running efficiently and prevent dust, dirt, and other airborne debris from accumulating. The most common filter levels sold in retail stores are MERV ratings from 1 to 16. These range from filters that can catch a golf ball (slight exaggeration) to filters that catch just about anything. People in colder climates may need to change their filter more often, as more air passes through the filter and it can become clogged more quickly. Without a filter, your furnace would stop working fairly quickly due to dirt buildup. To make it easier to remember when to change your filter, many manufacturers recommend changing it at the start of each new season.
A 5-inch filter will require fewer replacements than a 16x25x1 one. The lifespan of a filter is also reduced if your home's heating and cooling equipment is heavily used. Air is recycled more frequently, so the filter has a better chance of absorbing contaminants and becoming blocked. The filter manufacturer should indicate the recommended replacement frequency in the product information included when purchasing a new filter. If you have poor indoor air quality, your furnace filter won't last as long as it would in a home with acceptable air quality. Filters with higher MERV ratings are denser, so they require more power from your furnace to get air through them.
Not all furnaces have enough power for this. Changing your 16x25x1 furnace filter regularly is an easy way to maintain good air quality in your home, keep your furnace running smoothly, and even save money on energy bills. Marking the calendar every 90 days or trying to remember when you last changed your filter can be difficult, so following the manufacturer's recommendations or changing it at the start of each season can help ensure that you're replacing it at the right time.