Final answer:
White sauce should be boiled for about 2 minutes after the milk has been added.
Step-by-step explanation:
When making white sauce, also known as béchamel sauce, the sauce should be boiled for about 2 minutes after the milk has been added. Boiling the sauce helps to thicken it and cook out any raw flour taste. It is important to stir the sauce constantly during this process to prevent it from sticking to the bottom of the pan.