If you are a beginner the best way to judge would be to literally measure the milk. Say you started with 750 ml of milk as mentioned in the recipe then literally reduce it till reaches 500 ml which you can measure using a measuring cup. The trick for the same has been explained in this video. If you do not have a measuring cup and feel that the milk is too thick then add little milk to thin it down and if it is too thin then simmer it for a little longer to thicken