Combine the milk and sugar in a heavy 4 qt enameled cooking pot (4 Lt.) Make sure you use a cooking pot that is large enough so that it won’t boil over and spill. 

Heat over medium heat while stirring constantly. Once the milk comes to a full boil add ½ tsp. of baking soda and turn off the heat. The mixture will bubble and foam so be careful that the mixture doesn’t boil over. Keep stirring to keep the bubbles to a minimum. Once the milk has settled down, turn the heat back on the stove and let the mixture return to a simmer while you stir it. Start a timer for 10 minutes and allow the milk to simmer. 

As the milk continues to cook, you will notice that you can let it simmer without it burning. Keep checking on the milk and stirring, but you can let it simmer for a minute or two without stirring. When your 10 minutes is up, start your timer again. 

I find that checking on the dulce de leche every ten minutes is a great method for keeping an eye on the mixture. 

It takes about 1 hour to get the dulce de leche to 225°F (107.2°C) and about 90 minutes to get it to 235°F (112.7°C) But bear in mind your stove, your cooking pot, and your cooking flame will change the cooking time. Just keep an eye on the temperature.

The dulce de leche is fully cooked when it reaches a temperature between 220°F (104.4°C) and 235°F (112.7°C). As the dulce de leche continues to cook, the mixture will become darker and thicker. Cook to your desired degree of doneness.

My personal preference is a spoon candy cooked to 225°F (107.2°C) which has a rather blond color. However, a thicker version cooked to 235°F (112.7°C) is better for cookie fillings.