I boiled 4 cans for 5 hours & got perfect results. I removed labels from sweetened condensed milk, set them on their sides in a big thick bottomed pan, covered them with hot water to cover at least 1" over the tops of cans, and covered with a lid. Simmered them for 5 full hours, monitoring water level, adding water as needed. Let it cool in the pan overnight (I'm very patient), and next day I opened one can to find ooeeyy gooeeyy caramel. Still have opened can in fridge (minus small tasting) & 3 in the pantry for future use. It was almost too easy, but not quite as good as the regular caramel I've made before, the one where you cook sugar till browned & add cream. Thats a more volatile process but very good, and there are options that make caramels you can cut into squares, etc.