A simple recipe that requires little tending
Allow to simmer (not boil) for 40 minutes. Milk will thicken and darken a little. When done condensed milk should thickly coat a spoon and measure about two cups, but that measurement doesn't have to be exact. If it's not quite thick enough and you need to give it a little extra time, do! No harm will come from letting it simmer a little longer. Milk will also get thicker once it's placed in the refrigerator.