From the National Highway Traffic Safety Administration:

Although the average metabolism rate for moderate drinkers produces a .017 per hour decline in BAC level (here termed “Average”), and the average metabolism rate for heavy drinkers (who consume 60 drinks or more in one month) produces a .02 per hour decline (here termed “Above Average”), the range of metabolism rate in the population can go above .040 and below .010. One can either utilize in the calculation the average (.017 per hour decline) metabolism rate, or if one wished to use a very conservative figure, (which less than 20 percent of the population would exhibit), one could use .012 per hour decline (here termed “Below Average”) [Note: the BAC Estimator program provides BAC estimates for above average, average, and below average metabolism rates automatically. These three categories of metabolism rate closely approximate a drinker's recent drinking pattern, i.e., frequency and quantity of consumption.]

If we wish to be sure that we can determine when our blood alcohol concentration level has returned to zero, it might be well to use this very conservative (below average) figure.

From the Brewers Association:

The rate at which alcohol is absorbed into the bloodstream and subsequently metabolized depends on the individual. Some things to be aware of:

• The amount of food in your stomach. Food will slow alcohol absorption but will not prevent intoxication.
• Mental and physical health. Illness, depression, stress or fatigue can increase the effects of alcohol.
• Percentage of body fat. Alcohol will affect a well-toned individual less than someone with a higher percentage of body fat — even if both people are the same weight.
• Medication. Medication can increase the influence of alcohol. Follow your doctor’s advice before mixing the two.