Details
The Wunderbag™ is capable of holding MORE than 40 LB of fine dust, but the vacuum is designed to be emptied at 40 lbs. so that at least half of the filter bag's surface area is always free to support the high airflow rates required for effective dust control. The Wunderbag™ should be replaced whenever the vacuum collects 40 lbs. of dust, or when it appears to be half full. It should be replaced after being used for Wet vacuuming so the interior of the vacuum does not become moldy. It does retain its strength when wet, so it is not imperative to do it right away.
The HEPA filter's life can be greatly extended with the use of a Wunderbag™. The Wunderbag™ captures particles down to .5 micron, meaning that it captures 99% of the dust, leaving only the smallest particles for the HEPA filter. How often you change it is a factor of how often you use a Wunderbag™ with it, and how often you use it. Change it whenever you can see a distinct visible color change of the white folded filter material inside the cartridge.
The HEPA cover filter also captures dust down to .5 micron, providing a second layer of very effective filtration to protect, and extend the life of the HEPA filter. This filter should be replaced when it changes color and no longer looks white.