When you clean your bearings, inspect them for wear and tear. I would say to replace your bearings at least once a year ... just before the racing season starts. When you tear your car down for inspection, check the bearings and reoil.
Always keep extra bearings for your clutch bell (flanged and regular) since these take the most abuse from the engine. For the bearings in the hubcarriers, clean those after every day of racing. Lots of grit can get in there and destroy them.