Filters in betta tanks - how often to change?

[BettaMomma]

How often do you all change your filter cartridge on a filter like the whisper?
The one in my girls' tank is FILTHY - it's just gross, dark black.
I rinse it with dechlorinated water every now and then, but at what point do you just toss it and put a new one in??

 

[Synirr]

I change them when they get so disgusting I just can't stand it anymore or when they start blocking water flow, no matter how many times I rinse them out