up to 6 months. so it can go dead earlier also. the device would probably indicate when the filter would need to be replaced. and yes, it sounds fine. air purifiers need filter changes. and based on how bad the air is and how much it is used, you would need to replace them. the problem is that most stores advertise and sell only the devices. no idea how much the filters for competitive devices cost. xiaomi at least has been forthcoming with the pricing. availability to raam jaane.