Place the ingredients in a medium-large saucepan and use a silicon spatula to combine. Place the saucepan over a low heat and set the timer.
Cook for the full 90 minutes, stirring intermittently with the silicon spatula. I find a spatula best for this because it allows you to sweep the bottom of the saucepan and prevent burning.
Depending on the milk you've chosen, the liquid may change colours as it condenses. It should reduce considerably in volume, and (again, depending on the milk) start to look viscous. It should be able to coat the back of a spoon, even lightly.
If, at the 90 minute mark, the liquid still appears milk-like in consistency, keep going. There is no definitive time because stoves and pots are all very different. The benefit of using a plant based milk is that, provided you're stirring often and it's on a low heat, it's harder to burn or overcook than dairy milk.
Once the milk is condensed to your liking, take it off the heat and allow it to cool. Store in an airtight container in the fridge. It should thicken considerably once cooled, milk dependent. It keeps well for a couple of weeks.