The Quick Answer
The general barroom rule of thumb is that an average person can process about one beer or one shot of liquor each hour. Any more than that and alcohol collects in the bloodstream and intoxication ensues. But, though that generally holds true for everyone, several distinct factors contribute to the rate at which alcohol is metabolized in the body, from sex and size to genetics and frequency of drinking, making everyone's speed a little different. The contents of the stomach and the health and efficiency of the liver, the main organ in alcohol metabolization, will also determine the rate.
Alcohol in the Stomach
All foods and beverages that are consumed begin to enter the bloodstream in the small intestine, so naturally a full stomach will slow the rate at which alcohol reaches the blood. While this will make the effects of the alcohol more gradual, it also increases the time until the alcohol is effectively metabolized. Foods of high fatty content, or combinations of different types of food, like carbohydrates and protein, increase the time it takes for alcohol to leave the stomach and be absorbed. And, though the liver does most of the work metabolizing alcohol, the process does begin in the stomach with an enzyme called ADH. Women have less ADH activity in their stomachs, which means more of the alcohol reaches the bloodstream.
Alcohol and the Liver
Once alcohol reaches the bloodstream it generally metabolized at a rate of .015 of blood alcohol concentration (BAC) every hour. But, because BAC is a measure of proportion, it will take more drinks to make a larger individual reach a given blood alcohol concentration, and the amount of alcohol that is metabolized to maintain a rate of .015 per hour will also be larger. About 10 percent of alcohol leaves the body through the breath, perspiration, and urine. Almost all of the rest is metabolized by the liver. The usual fuel for liver cells is fatty acids, but in the presence of alcohol the liver will modify its diet to prevent a harmful buildup of alcohol that would damage other cells and organs. There is some evidence that normal alcohol intake makes the process more efficient to a point, creating a tolerance, though, in the long run, the metabolizing of alcohol permanently alters liver cells and decreases their ability to carry out normal functions as well as metabolizing alcohol. The accumulation of unused fatty acids can lead to cirrhosis, or fatty liver, and even liver failure.