As soon as the mixture is blended, turn on flame and keep stirring while you bring it to boil.
If the sauce is destined to be cooked in the oven, it’s enough to let it boil only 2-3 minutes.
If instead, it will be used straight off the stovetop, let it simmer at least 8-10 minutes. If not, you will still have a starchy taste from the flour, and you’ll find the sauce too sticky.
If not simmered long enough, béchamel will stick to the palate!