Steevo,
I only service them when I think they do not feel right any more.
Don't touch it if it is OK.
I think about 8 races could be OK.
But it really depends what the conditions are.
If your diff does not slip too much, and it feels smooth (not gritty) then it is OK.
If you've doubts about any of these, then check the diff.
rom Atlas there is somekind of diff protection sheet/sticker available for the TA-04.
I'm thinking about ordering it is pretty cheap.
Also one more thing that is important.
When you do run hot mods, renew the tiny locknutin the diff once in awhile, then the diff settings should stay more consistent.
good luck to ya.
Pizza's all the way baby