What is the Zakat nisab for money, and is Zakat paid on what a year has passed over, or on all the money accumulated from the monthly salary for 17 months?

Zakat is obligatory on zakatable wealth if it reaches the nisab (threshold) and a full hawl (lunar year) passes over it. The default is that each salary payment has its own independent hawl. However, it is permissible for the wealth owner to pay Zakat on all of his wealth by considering the hawl of his first nisab. This is better and for the benefit of the poor.