Just asking Mrs cortilla about this one (a doctor), and she says it takes 1 hour per unit for alcohol to leave the body, + 1 hour to the total (for an average person) for the body to start metabolising the alcohol (so if you take 1 unit of alcohol it will take 2 hours to metabolise and 3 hours for 2 units and so on)
As an aside the way i understand it, british law is a precedent law. So if a judge in a similar case at the same level or lower court has set a scentence of 6 months and that was the last case, then the scentencing judge would have to set the same scentence to remain consequent with the legal aspects. Only a higher court can reduce that scentence ie the court of appeal or the house of lords, but these will only look at cases on points of law, and not re examine the facts.