Depends on a lot of factors. Weight, metabolism, liver function, etc... As the body processes the alcohol, dehydration sucks the brain fluid low causing the headaches and reduces the amount of electrolytes in the system. Generally, 12-24 hours to recover. If you take some aspirin, and eat a couple bananas, drink some tomato juice and about 2 quarts of water you may feel better in about 4 hours.