Is zakat due on a plot of land inherited since its ownership, or since the intention to sell it?

There is no zakat on land so long as the intention of selling it is to dispose of it, not for trade. It does not become a trade commodity unless the intention of selling it is for earning profit. Zakat becomes obligatory on its price if it reaches the nisab and a full year has passed on it, at a rate of a quarter of a tenth (2.5%).