Add the coconut milk and sugar to a saucepan and heat over medium heat. Whisk constantly until the sugar dissolves and bring the mixture to a boil. Once boiling, reduce it to a light simmer and cook for 1 hour and 15 minutes until it thickens. Stir every 10 to 15 minutes so a film does not develop on top. You want the mixture to reduce by about half and be slightly thick – it will thicken a lot more once it’s in the fridge.
I find that this homemade version is as thick as store-bought condensed milk, however it doesn’t stay quite as “runny” – maybe due to the lack of preservatives? You can stir in a drop of milk to make it a bit more pourable for coffee.