The average person gets rid of alcohol at the rate of roughly one unit per hour - equiv to about 1/2 pint normal strength beer (proper beer - bitter) however, this is extremely variable and depends upon a number of factors such as body weight, metabolic rate, whether a person has eaten or not and if so what. It will also depend upon the rate and quantity over which the alcohol has been consumed. I have seen numerous back calculations carried out and the range is quite wide.
During less PC days I once attended a lawyers v cops christmas charity quiz held in a police station bar (since closed by a Chief Constable with no sense of fun). One of the events of the evening involved three individuals being taken down to the cells and put on the intoximeter. A sweepstake was then run on their readings. The results were quite surprising - a couple were in the expected range but the third registered about half what he should have done but then he was a fat bastard who had consumed a donor and chips early on in the session.