Storing bechamel sauce
You can store béchamel for 4-5 days safely in the refrigerator.  Cool the unused portion as quickly as possible after the sauce is finished and make sure to place a piece of cling film directly in contact with the surface of the sauce prior to refrigerating it so that it doesn't form a skin and slows down oxidation.  Do not freeze the sauce as it will likely break the emulsion.

Another way to make it easier to make your nightly béchamel would be to pre-make a larger batch of roux and freeze it in portions for later use in sauce.