Genuine Dyson DC26 washable post filter

When would this part need replacing?
-After the motor has failed
-When the pre filter has not been washed often enough causing this filter to block too.

Fits The following DC26 models:
DC26 Multi floor, DC26 Multi floor plus Carbon fibre turbine head, DC26i, DC26 City

Genuine Dyson DC26 HEPA post filter 915219-03