Importance of replacing filters:

When to replace: regular use in the home, usually good for about a year. During wildfire season, so much smoke. Likely going to need to change it during that season. As it gets dirty, replace it. As it fills up, air will find a way through. If it's full, dirty air will go through the side of machine. Stock up on at least one extra HEPA filter for smoke season. 

Where to buy: