Alms Tax: Maintaining the Nisab for Eleven Months
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1918
As stated earlier, zakat on gold and silver becomes obligatory when a person maintains ownership of their niṣāb for eleven months, and if during those eleven months the amount of gold and silver falls below the first niṣāb for each of them, then zakat is not obligatory on them.
-Ayatullah Sistani, Practical Laws of Islam, Alms Tax (Zakat)