When your wealth is equal to or greater than the Nisab after one lunar year, you are eligible to pay Zakat. That can be hard to determine especially for those of us who have multiple streams of income and different amounts coming in throughout the year which makes it harder to keep track.
Pick one day out of the year and calculate and pay your Zakat on that day every year.
Typically, people prefer to pay their Zakat during the month of Ramadan. It is not prescribed but giving charity during this blessed month guarantees added rewards.
Zakat can be pre-paid before your set Zakat due date. However, if you do pay early, make sure to calculate your Zakat on your due date to confirm you have paid the right amount.