I ran my new and totally stock B5M indoors several times (maybe 8-10 battery packs of run time, maybe 20-25 mins each) and the bearings still felt fine. The track was indoor fairly hard clay, kept a bit damp and not dusty. Then I ran one race outdoors (3 heats + main) on a very dusty track and my bearings on the trans output shafts were pretty crunchy by the end of the day. I pulled the trans apart and sprayed the bearings out with WD40 until they felt smooth, and then oiled them back up with light weight oil. They are back to smooth again (4 more packs run) and I seemed to have saved them so far... The rest of the bearings on the car are still fine without any care. That said, I don't think the stock bearings are sealed very well and I doubt they will last very long if you run on very dusty outdoor tracks. I've never tried any aftermarket bearings, but perhaps some have better seals.