How do I know when it's time to replace the Pre-Filter versus the HEPA Filter?

When the Filter Check/Reset Light illuminates with a solid light (not blinking), it is time to replace the Pre-Filter, which should be done every 3 months. When the Filter Check/Reset Light blinks every 2 seconds, it is time to replace the HEPA Filter, which should be replaced every 12 months. These replacement intervals are guidelines only and may vary based on home conditions. (Page 6–7)