Guardian air purifier doesn't indicate that it's time to replace the filter.

The air purifiers that are equipped with a replace filter and/or UV bulb indicator have an internal timer that let you know when you should replace the filter/bulb. For the filter, the timer is set to go off every 6 months or so (if running 24/7). For the bulb, the timer is set to go off every 10 months or so (if running 24/7). This is based on continuous run time. If you run the air purifier only 2-3 hours a day, it's going to take a lot longer than 6 months for that indicator to go off because you are not running it 24/7 and the internal timer is counting down only when the unit is in use. 

The unit cannot determine if the filter is dirty or if the bulb is burned out. Your filter may be dirty and need to be changed before the indicator goes off or if may still be clean when the indicator goes off. We recommend checking the filter monthly, vacuuming off the pre-filter and checking for signs that your filter may need to be replaced (the white part of the filter starts to turn grey in color) You can also reference this article for help determining when you need to replace your filter. 

If you do replace the filter before the alert goes off, you will simply need to reset the alert (you can find information on how to do that based on the model number by clicking here.)

If you are wondering why the alert has not gone off yet, you may have not run the unit for the full amount of time yet and that the alert should be going off soon.