You remember that sweet carbon smell you got when the Filter was new??

That's what you're looking for.. chances are if the filter has been running 24/7 for 3 years it's long past due for a changing.

There's no exact life span on Carbon Filters because the usage can vary so much.. some run it 24/7, some run it only when the lights are on...

Either way, think back to that sweet carbon smell that you got when the filter was new... When you don't smell that anymore.. it's probably time to change it.