Regularly replacing these will help to prolong the life of your filter by keeping it from getting clogged up with larger particulate, and ensuring your filter is working efficiently.
As a rule of thumb - if it looks dirty then it needs replacing - just select the correct size for your fan.
Replacement outer sleeves for both the Rhino Pro and the Rhino Hobby filters. As they are slightly different sizes, you will need to select your filter type from the dropdown.
Generally speaking, if things look dirty, they need replacing!