Pour the sweetened condensed milk into a heavy-bottomed saucepan.
Place the saucepan over low heat.
Stir the milk constantly to prevent scorching.
Continue stirring as the milk heats up and begins to bubble.
Let the milk simmer gently, continuing to stir for about 1-2 hours.
Watch for the color to deepen to a rich caramel shade.
Once thickened, remove from heat and let cool slightly.
Transfer the dulce de leche to a glass jar.
Let it cool completely at room temperature.
Store in the refrigerator for up to 2 weeks.