You'll need to boil the water for about 2-3 hours, or until the milk has turned into a beautiful, golden caramel. Yes, you read that right, 2-3 hours! It's like waiting for a pizza to be delivered, but instead of getting hangry, you'll be rewarded with a delicious, sweet treat. Just be sure to check on it every 30 minutes or so to make sure the water level doesn't get too low.
Now, I know what you're thinking: "Why can't I just microwave it?" Well, let me tell you, microwaving is not an option here, folks! It's like trying to make a cake in 2 seconds – just not gonna happen (trust me, I've tried). You need to let the caramelization process happen slowly, so be patient, young grasshopper!
As you're waiting, you might be wondering, what's actually happening in that can. Well, it's quite simple really: the lactose in the milk is breaking down, turning into a rich, creamy caramel. It's like a little science experiment, but instead of a volcano erupting, you get a delicious treat!
Easy Sweetened Condensed Milk Caramel at Michael Brehm blog