When club racing I usually just rebuild the diff whenever it begins to feel gritty.
It really will depend on your track conditions and driving style on how long your diff should last.
If you're going to a big race, I usually put fresh diffs in my cars before practice starts... and USUALLY that's enough to get me through the event. There are times however that I will replace the diff durring a large event, but that's usually just if it starts to feel gritty like I mentioned before.
Usually when I rebuild the diff, I will take the diff apart, check the large and small rings, the caged thrust, and the diff balls.
Sometimes all a diff needs is relubing, but that is not a guaranteed fix for a gritty/bad diff.
Usually I will flip all the rings, relube, and reassemble the diff and go from there.
To truely "rebuild" a diff you should replace all the rings, the balls, and the caged thrust assembly and of course relube everything.
Hope this helps.