IIRC, Mr Happy is right, it is one hour per unit of alcohol. Even if I am wrong and you are right, Dick, it does no harm to assume that.

Usually if I am likely to be out for two hours or more - typical folk club evening, I will have one pint. If it is likely to be less I will make do with a half and have a soft drink if I want any more liquid.

PS: I loved your mondegreen in your opening post "at least18 driver skilled in crashes beteween 2003 and 2005 had ablood alcohol level of between50 mg and 80 mg"