Metabolisation of alcohol depends on a number of factors, including weight, sex, age, fitness, food intake etc etc.
A very rough guide has been quoted, 1 unit is burnt off every hour or so.
Perhaps pertinent to flying is the detrimental effect alcohol has on the balance mechanisms within the ears, some figures I've seen, suggest that this can hang around for aproximately 48 hours, long after any detectable amounts are found in the ear.
Have a very Merry Christmas, and safe flying.