Check with the manufacturer. Some have their own Kitchen Scrubby & others don't say anything about it, so you buy very much more frequently. My understanding & experience is that you want to scuff or sand the surface visibly clean. Basically, sand off anything that's been caught, since really nothing gets past the outer most surface. The outer surface just gets more crowded.
I'm on my 3rd sanding with a Pro-Pur & did 15 sanding's on a previous Berkey. I still have the Berkey as a spare & it probably has 30 or more sanding's left in it. It was working fine & at the same "good for so many gallons capacity" with no weakness or threat of collapse yet, but I wanted to try the Pro-Pur. In both cases, my slow drip was restored to the original quick drip & no taste or smell difference & a food coloring test reveals no difference.