When do you replace a shop vac filter?

I know, this is not a home improvement forum, but the advice here seems better than I see on the DIY sites.

Went to vacuum the cars and decided to clean shop vac filter (Rigid brand). It uses the blue circle fabric filters for about $30 each at Home Depot.

Bought it and three filters about 10 years ago. I bang them against a fence post when they appear dirty and then rinse with clear water. Let them dry and than re-install. One developed tears, so I threw it out. The other two (10 years old), have lost some of the blue coating -- exposing white fiber underneath -- but otherwise appear fine.

Do I just wash and reuse until they tear? Or, since the blue outer layer has started to come off, is it time to replace.

I really have gone through a bunch of Web sites, but have not found an answer (I have ignored filter maker sites).