Rule of thumb. This is a surprisingly complicated question, as it turns out, in terms of physiology. To make a long story short, the rule of thumb is that it takes about one hour for the average person to clear one "standard drink" (about 1/2 an ounce of ethanol). This equates to 12 ounces of beer (3.2%), 5 ounces of wine and 1.5 ounces of whiskey.
Short time. We metabolize about one drink per hour, faster in chronic drinkers, and slower in people with liver problems, which can be caused by alcohol. Most alcohol is metabolized in about 12-18 hours. Metabolites of alcohol can stay in blood and urine for much longer, up to 72 hours or more. The toxic effects of alcohol, however, can last indefinitely - the damage it does can be permanent.