The site is not used up, but only Jacob has access. If he is killed, you'll need to "use" it for the others to gain access.
Wait, but I thought when you are in battle with Jacob, And even if he dies It would not have to be used up once, because it was alrready in the battle without not having it to be used once?
Nope. Promised Land only gains unlimited site access to Jacob.
Promised Land
Type: Site • Brigade: Multicolor • Ability: None • Class: None • Special Ability: Hero gains access to any site. Abraham, Isaac, Jacob, Joshua, and King David may use Promised Land without limit. May only be used twice by other Heroes, then it must be discarded.