The standard text book answer is that 1 unit of alcohol takes, on average, 1 hour, to metabolise.
(Hence 6 pints of Boddingtons is not cleared from your system for about 12 hours.)
However, pilots are a bit of a special case, as there is some evidence which suggests that alcohol can stay in the fluid filled tubes which are responsible for balance, for about 3-4 days.
I guess its horses for courses.