Originally Posted by bigMikeB
When ever it is dirty, there isn't a specific time that works for every installation.
If you're in a house that has smoke and multiple pets, multiple kids, etc, you'll go through them way faster. I've had them last for practically no time at all (but that was because I was sawing through tiles inside (to accomodate a threshold piece) and the saw had no dust controlling features, so my home was thick cement/tile dust. That covered the entire place and plugged the filter up nicely.