Impellers keep your filter running smoothly by creating the suction that draws the tank water into the filter and through the filter media. With proper maintenance, impellers should last quite a while, despite being very fragile items. Proper maintenance includes cleaning the impeller of any debris, as well as cleaning the filter magnets and housing to reduce friction and pressure on the impeller.
Inspect the impeller for damage every time you clean it; if you notice any cracked or missing blades, replace it immediately.
Replace them: once any signs of damage occur.
For more information regarding when to replace aquarium filter equipment, see our blog post.