بسم الله الرحمن الرحيم
After turning sahib-e-nisab when one complete year passes over the wealth then it is wajib to pay zakah of that wealth. And that time i.e. at the time of completing one year whatsoever zakatable amount is in your hand you will have to pay its zakah after deducting debts if any. Zakah becomes fardh only once in a year on wealth. It is not so that a person who is sahib-e-nisab has to pay zakah each month on his salary. If you think like this then it is not correct concept. If you save money from your salary and did not spend it in your needs rather it remained with you then on the zakah day it shall also be included with the nisab and the zakah of total amount shall be paid. If that amount remains with you in the second year as well then in the second year too it shall be included with the nisab and zakah will have to be paid.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.