In a 3- to 4-quart pan, combine 2 cups sugar, 2 cups water, and 3 quarts rinsed fresh mint leaves (about 4 bunches); stir often over medium-low heat just until simmering. Let stand until cool, about 30 minutes. Pour through a fine strainer into a small pitcher. Chill until cold, about 45 minutes. Mixture keeps, covered and chilled, up to seven days.