@jeff1047 That's a great question. I can say that my experience with the Foust 160R2 model is that I have usually changed the carbon in it every 9 months to a year. I have also changed it at 6 month. By a year, I can tell that the air doesn't smell as fresh, and I've wondered if some of the dust held in the filter can be shed from the edge of the felt if it gets overloaded. It's not just dust in the air, but also dust that comes off the carbon. This answer needs some context.

The cylinder is made so there is a one inch wide chamber in the perforated wall to hold the carbon granules, and it holds nearly 7 pounds. I repack the cylinder myself which is a very messy dusty job, so I always do that outside and I'm careful not to inhale any dust. Wearing a respirator is advised. That is the most economical way to change the filter. You can also get a pre-packed cylinder shipped to you and return the old one. They are re-packed and resold.

Inside the cylinder is an airspace, and the "filterdown" or HEPA felt material is held against the carbon chamber with springy metal strips and hardware cloth. The HEPA felt is an additional charge, and I always buy that and also get the filterdown they give you for backup supplies. It is easy to cut yourself on the metal springs or hardware cloth while changing the filter media. The industrial type motor on the top lid is connected to the bottom lid by a long bolt with a wing nut on the bottom. Air is drawn first through the carbon chamber then through the filter felt fabric and upward from the center out the top of the unit's blower. If you want to use the filter down without carbon, you can do that. They will also sell you a pleated filter that can go into the carbon chamber if you don't want to use carbon.

I have had these units last a long time (8 to 10 years) before the motor might quit. There are no user serviceable parts, and the bearings can wear eventually causing the motor to get hot. Cooling for the motor is only through some air vents and dust can collect in there which needs to either be brushed out or blown out with air for better air cooling. It all depends on how much you run the motor. We run these most of the time. The unit in the bedroom was run only when in that room at night. I no longer trust a unit that gets hot, so I run that only about an hour before going to sleep and turn it off. When we are away from home, all units are off.

We also have the larger 400 series running all day on the main level of the house. It has a prepacked filter box that lasts a few years. All you have to do is change an outer filter much like a furnace filter that catches the large dust. You can also get this with a UV light, and I've had that bulb burn out after a year, but the filter is fine. I think I've had this for at least 4 years, and the air coming out the top still smells fresh, and I have not replaced the original filter except for the outer one. This unit costs more, but it is so much easier to keep up. Removing the filter is just lifting off the top and lifting the box out. That filter cannot be re-packed and you must buy a replacement. There is no mess. The unit is on casters, so can be moved around. When the box arrives, it's too heavy for one person to lift, so I opened it on the driveway and took the pieces inside individually. This also has a larger industrial motor inside the unit so you don't see it except when removing the outer filter.

I think the company does represent the life of the filters honestly. These are made without glue or adhesives because of people with sensitivities, so they may not be the most attractive air filters you can buy. My 400 series is a solid beige color. The 160R2 units are a metal cylinder (which can rust) with a black industrial motor on top. The 400 series is nicer and easier to maintain, but if you are buying replacement pre-packed cylinders for the 160R2, all the dusty work is done and you just swap out that cylinder. In the long run, filter changes may cost less over a long period of time with the 400, but the unit is more expensive. There are 2 fan speeds on the 400, but they are nearly identical, and the unit is fairly quiet. The 160R2 is a bit louder, but not bad. These filters do a good job of removing dust, smoke, pollen and chemicals like VOCs, fragrances and formaldehyde that may be in home construction materials. On the days when outside air quality is poor due to far reaching wildfire smoke, the air inside my home was safe to breathe. If you have pets, that may clog the filters sooner. They do sell a cover to put over the cylinder on the 160R2, and I tried that, but the motor burned out right after that, so I'm not sure I would recommend doing that and adding an extra load to the motor.