The amount of alcohol required to get drunk depends on a number of factors: the size of the person drinking, the size of the drinks consumed and the frequency with which they're consumed, among others. Furthermore, different areas have different definitions of legal intoxication (though a BAC of .08 tends to meet the criteria for inebriation in most parts of the United States), which further alters the criteria. The better you understand the different variables involved, the more easily you can determine how much it takes to get drunk.
Size of the Drinks
The United States government defines a standard drink as .6 ounces of pure alcohol. Different beverages contain different percentages of alcohol, so the type of drink has a bearing how quickly you get drunk. The general rule of thumb equates .6 ounces of alcohol with 1 12-ounce bottle of beer, 1 full glass of wine (about 5 ounces) or 1 shot of hard liquor (about 1.5 ounces). Each drink creates a blood alcohol content level of about .025 or .03 once consumed. Three such drinks in the space of an hour raises the blood alcohol level to the legal definition of drunkenness at .08.
Size of the Person
Beyond the size of the drinks, the individual's size and mass has an effect on the equation. With more body mass to absorb the alcohol, a bigger person needs to drink more in order to achieve the same effects. As a rough guide, every 20 pounds of body mass reduces the blood alcohol content by .01 for each drink you take. (That number increases dramatically the more alcohol you consume, but only at levels which qualify as legally drunk regardless.) In addition, women, who have smaller mass and a higher content of body fat, tend to become inebriated on less alcohol than men.
It takes the human body about one hour to process one drink: in other words, the body gets rid of the alcohol much more slowly than you can take it in. You can mitigate that number by taking in food when you drink, which slows the rate of absorption and keeps you sober longer. In addition, while drinking water doesn't slow the process of inebriation, it keeps your body hydrated (reducing the effects of a hangover) , and switching drinks of alcohol with drinks of water slows the rate at which the liquor enters your body. (Some people claim that not mixing drinks will slow down the rate at which you get drunk, but alcohol is alcohol, and the effects remain regardless of how you ingest it.)