I do a light cut of the comm after a few runs, just enough to bring the proper colouring back and stop it getting too burnt, I'll then re-serrate the brush and put it back in, I only replace the brushes if they get burnt, look 'chipped' etc on the face or they reach 8.7mm in length (from the back end of the brush to the end's of the curved section).
At this length, when on 40MHz, not long after reaching this length I would almost always start to get glitching etc.
Skiddins