Asked by: Fergus Collins, by email
For most people, a single unit of alcohol takes around two hours to metabolise. For ordinary-strength beer, the legal limit is approximately two pints, so you would need four hours to metabolise the extra two pints.
However, this should be regarded as an absolute lower bound; lots of factors can increase this time. A smaller person will metabolise alcohol more slowly and be affected more strongly, many beers can contain as much as 50% more alcohol than ‘ordinary’ strength and age, sex and diet all have an effect as well.
As a rule of thumb you should always reckon on sleeping off your alcohol before you drive and after a very heavy night even that may not be enough. Ten pints of strong lager could take more than 24 hours to fully metabolise.