Add milk and sugar to a saucepan or Dutch oven, turn heat to low, and allow the sugar to dissolve completely.
Once the sugar is completely dissolved, increase the heat so that the mixture comes to a low simmer.
Simmer on low, undisturbed, for 40-45 minutes until decreased by half.
Strain mixture through a sieve into a bowl or measuring cup, transfer the liquid to a glass jar with a tight fighting lid, and then store in the refrigerator. Condensed milk will stay fresh, refrigerated, for approximately six months.