How long can pasteurized milk be refrigerated?
Per Still Tasty, well refrigerated, milk should last about 5-7 days.  Note that this almost certainly assumes that the milk has been pasteurized.

It is theoretically possible that you could periodically reboil milk, and then try to get it cooled to refrigerator temperatures rapidly, to increase its life.   I suspect that the benefit would not justify the effort.

You can also freeze milk, if you have a freezer available.  It will last essentially indefinitely (there probably is a limit due to oxidation of the milk fat, but that is a slow process), and will still be quite suitable for cooking.

Depending on availability and how you use it, you may also wish to consider shelf stable milk products like dry powdered milk, condensed milk, or sweetened condensed milk.