Prepare condensed milk: Pour the milk into a medium heavy saucepan and bring to a boil. Reduce to a simmer and cook 45 minutes, stirring occasionally. Stir in the sugar and continue simmering 10 to 15 minutes or until reduced to 3 cups. Strain, Homemade condensed milk can be refrigerated up to a week.. Meanwhile, prepare the mate: Heat a teapot, rinsing it out with boiling water. Add 1 tablespoon mate for every cup of tea desired. Pour boiling water over tea leaves and let steep 5 minutes. Strain and serve with condensed milk. (To drink cold, use twice as many tea leaves and pour tea over ice cubes. Serve plain or with sugar and lemon.)