How often do you change your carbon pellets?

[Tbg299]

Hello, for about 3 weeks I have been running a BRS deluxe carbon reactor in my sump with ROX 0.8 carbon. How often should it be replaced with clean carbon?

 

[NowGlazeIT]

I use that carbon. I replace it when I notice the flow slowing down to a crawl. I only get about 3 weeks out of my brs black reactor