As a rule, I use 1 tablespoon of cornflour to thicken every ½ litre of liquid. I mix the cornflour with a little cold water, and then stir it into the hot food during the final stage of cooking.
It must be brought to boiling point before thickening begins. At that point, it usually thickens fairly quickly and the sauce turns from opaque to transparent. However, sauces will thin if cooked too long or boiled too vigorously.