My furnace has been running daily. I hear you should change the filter about once a month.

My filter still looks clean. Should I change the filter when it looks dirty or about once a month when you are using it regularly.

Thanks in advance.

Once a month is just a general rule of thumb for most cheap throw-away filters. Truth is that everyone's application will vary due to certain things, such as pets for example. If it looks "clean" then don't worry too much about it.

Mark is correct. It is best to check filter once a month, but only need to change it when it is dirty. May be one month, or three, or six depending on environment or furnace.

If you check the filter, and it appears to be clean, then check it again in another month.