Plan on him serving the full stretch. DWI convictions are not getting much good time from the parole board. If the "aggravated" portion was added, it is almost a certainty.
I agree with Mr. Case. The parole boards often require serve alls (serve all of the sentence without release on parole) on alcohol related offenses. If he also got an assault with a deadly weapon charge (perhaps intoxication assault?), then he will be required to serve 1/2 of the sentence before even being eligible for parole.
Good time conduct cannot be guessed upon without a LOT more facts (and even then, it is very difficult unless you consult with a parole lawyer who works on those kind of situations on a regular basis.)