Check on it constantly to make sure the water level isn't below the can, and that the can is submerged several inches.
Set a timer to check on the water every 20 minutes!
Quantity: you can cook more than one can at a time depending on how large the pot is. Always make sure there's a lot of room in the pan, and the cans are covered by water the whole time.
Water: ensure the can is covered in water. Set a timer to check on the water every 20 minutes!
Cool down: make absolute sure the can of condensed milk has cooled down completely before removing from the water and opening it.
Cooking time: cook from 2.5 hours to 4 hours. The longer you cook, the thicker and darker the dulce de leche will be. For a light and runny dulce de leche, cook for 2.5 hours, for a dulce de leche that is easy to pipe and holds its shape, cook for 3.5 to 4 hours.