How much alcohol percentage in 55 proof brandy?

Alcohol, by definition, is 100% alcohol (200 proof). A 55 proof liquor is 27.5% alcohol.

The amount necessary to get you drunk depends on your sex, weight, how fast you drink it over what period of time, and whether your are accustomed to drinking. Different people also have different tolerances: alcoholics can drink more than beginners, without showing it, because they have developed a tolerance, and also because they have more practice. They are, however, just as drunk.

That said, most folks would reach the legal limit for driving with less than 10 ounces of 55 proof liquor consumed within one hour. Depending on other factors, they might even be staggering.