1- Boil a deep pot full of water.
2- Remove the wrapper from your condensed milk tin. Dont open the can!
3- Drop the condensed milk in the pot and keep the lid closed. Allow it to boil undisturbed in a medium heat for 2 hours. 4- 2 hours later, allow the tin to cool down and then open the can.
5- You can store it in an airtight container for up to one month in the refrigerator.
When the water begins to reduce keep adding boiling water to the pot. The can should always be immersed.
P.S Do this method if you have an induction cooker. If you don't want to waste the precious gas, Google the pressure cooker recipe for the same.