In a saucepan over medium heat, add coconut milk and sugar. Bring to a low boil, reduce heat and let simmer rapidly for approximately 45 minutes, stirring frequently. 

Keep simmering until the coconut milk mixture reduces by about half and is quite thick - you should end up with about 3/4 cup of liquid. Sometimes it might seem like the mixture is going to boil over. If this happens, remove the pan from the heat until the mixture goes down and reduce the heat before putting the pan back on the stovetop. 

Remove from the heat and let cool before using in a recipe or storing for later. For storage, place sweetened condensed milk in an airtight container in the fridge for up to 5-7 days.