Com Cutting!!!!
Man.. surprised by how much bad information runs through these boards!
Your motor can run many many times before it needs to be cleaned... but you dont want to! You should clean your comm and the faces of your brushes after every run. Even if you are just bashing. It only takes a couple minutes. Don't forget to re-oil your bushings/bearings.
I am a heavy competator, (i understand you just bash) and my comms are cut every 3 runs, and brushes are replaced every 15 runs or so. But after every 5minute race, the entire motor comes apart, the comm and brushes are cleaned to a mirrior finish, the can and bell are sprayed out, the shafts are polished with metal polish, the bushing are reoiled, and the can is put back together... every 5 minutes.
So, the more performance you want, the more you need to do. you COULD cut your comm and change your brushes every run, but that is the most extreme.
My advice for you, is to clean your comm and brushes every run. Cut your comm and change your brushes every 15-20 runs.