XRAY - Model racing cars

How often do you change the oil of the shocks?

Once one of mine gets air in it, I then top up all 4.
If the oil looks very grey or cloudy I refill completely.

I top up my XRAY aluminium shocks before every meeting to reset the rebound to 100%. Apart from that I rarely fully change the oil (I find these shocks,unlike the kit ones, do not let in air).

For every race or if there has built some air inside - then immediately.

I read in setup book If you are in long race, for example in two days, is too recommendable refuilt?

The T4 shocks are very good and don't leak that much. I'll usually do mine before an important race weekend, then get that weekend and at least a few club meetings out of them before doing them again. If they're leaking more than that then it may be time for some new o-rings.