My apologies if this has already been addressed in another thread, but I just wanted to know when people feel it would be necessary to replace an Austin filter? Mine are both new, and the company claims that "with normal use" the filters could last up to 5 years. Neither of mine will be used under "normal" conditions, as they are both in a room with at least two birds. IME, ordinary air filters that claim to last 6 months normally last 2 months under these conditions. So, how long can I expect the Austin to hold up? 2 years? Less?