Khums on Savings, Gifts, Mahr, and Monthly Salary - Jurisprudential Answers - Miscellaneous Issues, Vol 3

Ahmed al-Hasan Response:

Khums is due on money remaining beyond the year's living expenses. One fifth is paid once, and khums is not repeatedly taken from the same money if it remains for years after its khums has been paid. As for what you earn each month, you do not have to pay its khums every month. Rather, one fifth of the surplus is paid at the end of the year. Choose a day of the year for yourself, and on that day every year pay one fifth of the money remaining from that year's earnings. May God grant you success in every good, keep you firm upon the truth, and make you among those through whom He gives victory to His religion.