If your intention was to develop the land in order to live there or to rent it out, then there is no zakaah on this land because it does not come under the heading of trade goods. Please see the answer to question no. 129787
But if your intention was to deal in it, then the basic principle is that trade goods are subject to zakaah. So the value of this land should be worked out every time one hijri year has passed, then you should pay zakaah on it according to its market value.
But because this land has been usurped and you are not able to dispose of it, there is no zakaah on it according to the more correct scholarly opinion.
Ibn Qudaamah said in al-Kaafi: With regard to usurped property, lost property and debt owed to one who cannot get it off because the debtor is in difficulty or because he denies he owes anything or he keeps deferring the payment, there are two view on this issue… etc.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: There are two opinions in our madhhab. One view is that zakaah is due on it and it is obligatory, but he does not have to give it until he takes possession of it, whereupon he should give zakaah for the past period, even if it has gone on for ten years.
The second opinion is that there is no zakaah due on that, because the wealth is not in his possession and he is not able to ask for it, and if he asks for it he will not be able to get it. This view is the correct one.
End quote from ash-Sharh al-Kaafi.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: Zakaah is not due on a debt for which repayment has been deferred, or the debtor is in difficulty or is unemployed or is denying that he owes anything, or on usurped or stolen property. This is the view of Abu Haneefah.
End quote from al-Ikhtiyaaraat, p. 146
But to be on the safe side, if you regain possession of this land, you should pay zakaah for one year, even if it remained in the possession of the usurper for several years.
For more information, please see the answer to question no. 125854
And Allah knows best.