A furnace filter won’t last forever.  In fact, if you are asking how often you need to change it, the chances are you never have. If this is the case, then the answer could be—now! Still, even if it doesn't seem like long ago, there can be many benefits to changing a filter, and here is why:

Your furnace will have to work a lot harder if it is clogged up. Dust, fluff and other items can block your furnace meaning it will lose efficiency over time. This affects the furnace in its entirety and can lead to you needing to replace some of the more expensive parts. The worst-case scenario is that it reduces the overall lifespan of the furnace. Clean filters save you money on bills, and costly repairs. 

Quite simply, a clean filter gives you cleaner air. This is important for everyone, but especially for those who have breathing difficulties such as asthma. A dirty air filter can make any existing problems worse, so it is always best to check that you know the correct furnace filters sizes and get a new one if it isn't clean.

If your air conditioning system is continually breaking down, then your filter could be to blame. You shouldn't go longer than a few months between changing them so if you have left it longer you could be reducing the lifespan of your unit. We should note that there are higher quality filters available that last longer but even these should be changed every six months.

A big reason to change your furnace filter is that you have a cat or dog. Their dander can collect in the filter which blocks it up much faster than if you did not have pets. Given this, you will want to change it every couple of months. You will know when your pet typically sheds.  After a couple of months, you will get a feel for how long after this it's time to change your filter.

Furnaces will work harder when they are pushing air into a larger space.  Smaller homes heat and cool down a lot faster because the unit doesn't have to push as hard. The size of your home will impact how long your filter will last. Smaller homes might require smaller filters which are most likely to require changing.  You can take your filter out to inspect it today. The more prominent the layer of dust, the more urgent a replacement is. 

Most people leave it way too long between changes. This can result in health issues that were not present before. For the benefit of everyone that lives in your home, replacing the furnace filters will help everyone breathe a lot easier. This includes pets, who can be a big reason to replace them in the first place.