A Long Stay Price is the price per night if guests stay 28 nights or more.
Go to Prices > Price Update > Configuration to make sure this option is active. Then go back to the Price Update to load the price.
Once you've added the Long Stay Price, every time a guest books for 28 days or more, it will override the room price.
Remember: The Long Stay Price is always entered on a per night basis, regardless of the price basis.