When is Zakat due on saved money whose monthly amount is unknown, and is it permissible to pay Zakat on the entire sum at the end of the year?

Zakat is obligatory on saved money if two conditions are met: reaching the nisab (the equivalent of 85 grams of gold or 595 grams of silver) and the passing of a full hawl (a complete lunar year) on it after it has reached the nisab. If both conditions are met, zakat becomes obligatory, and its amount is 2.5% of the money. It must be paid immediately.