The typical life of a furnace filter is 3 months. This can vary based on how much the fan is running, how dirty the ducts are, and the lifestyle of the tenant/homeowners. Pet hair and dander can be a significant factor as well. The best way to gauge how often the furnace filter needs to be changed is to check it often and see how dirty it gets.
You should never let the filter get to the point when you hold it up to the light and only 50% of the light is visible looking through the filter.
A good rule of thumb if you live in Denver, CO is to replace the filter in the fall when the furnace first starts running a fair amount. Replace it again at the end of January and if you forget to check it again at least you are changing it twice a year and this should be good enough to make sure it is pretty clean. If you have a two stage high efficiency furnace the fan will tend to run more often so you may want to change it January first and again in Mid March.
If you have central air conditioning, then it’s a good idea to check the filter monthly in the summer to make sure it is clean. Changing every 2-3 months is usually sufficient.
It is more important to check and replace the filter than to pay extra for an expensive filter you only change once a year. Having a clean filter will extend the life of your furnace and can save you a significant amount of money by following a maintenance schedule.