There is no zakaah on this money until one whole year has passed since it came into the possession of the beneficiary. 

Al-Bahooti (may Allaah have mercy on him) said in Kashshaaf al-Qinaa’ (2/172): Zakaah should be paid on money that is bequeathed to a specific individual after it has been in his possession for one year. End quote. 

Shaykh Ibn Baaz (may Allaah have mercy on him) was asked about a man who died and bequeathed 2000 riyals to one of his brothers so that he could get married, and a few years passed between the man’s death and his brother’s getting married. Is zakaah due on this money? 

He (may Allaah have mercy on him) replied: If he bequeathed it to a specific individual, then it became the property of that individual when he took possession of it, and he should pay zakaah on it when one year has passed even if he delays getting married. End quote. 

Majmoo’ al-Fataawa (14/41). 

And Allaah knows best.