With steel balls I just change them every time I rebuild the diff. That's like every two races or so. With ceramic balls I think you can just keep cleaning them untill the diff isn't smooth enough when freshly rebuilt. I just started using the ceramic balls this carpet season and havn't had to change them yet. My team mate used them all last summer racing on asphalt, the diff stayed smooth for many races. When I built his carpet car last fall, I cleaned the same balls and used them again; to my surprise they were perfect!
Don't forget to check the thrust bearing when your diff feels gritty. It only takes one good hit to waste one of those bearings, and they wear out quickly without the hits, too.