How long does it take for a glass of alcohol to leave the body?

It depends: It depends on what kind of alcohol beer vs wine vs other alcohol.

8 hours: Alcohol has an approximate detection time of 12-24 hours in urine. People who achieve a moderate level of intoxication (0.10% blood alcohol content - bac) are detectable about 8 hours after achieving that level. The interesting fact is that alcohol is absorbed into the blood relatively quickly and metabolized more slowly. Moderation is the key.