For those running an Aprilaire air cleaner on their air handler (or similar...4" or 5" large surface filters) how long do you go between replacement filters? They all say "up to 12 months" but based on what they look like when I pull them out I end up doubting that is a realistic interval. I've run 9 months for the last couple but now am going to go every 6 months and see how that goes. We have a clean house that my lovely wife vacuums and dusts weekly, and a dog that doesn't shed so no pet hair. The blower does recirc and supposedly runs about 30% of the time. I suppose a static pressure test before and after a change would give some definitive answers, but on the other hand for what equates to about $100/year I'm thinking why not just go every 6 months going forward?