Zakat is due a year after your level of personal wealth has reached the Nisab threshold. In order to remain eligible to pay Zakat, your level of excess wealth should remain higher than the Nisab for a full lunar year.
For example, if your level of expendable wealth exceeds the total Nisab value in the month of Ramadan, your payment will not be due until the following Ramadan, so long as your personal wealth remains above the threshold.
If at any time your wealth drops below the level of Nisab, you would cease being eligible to give Zakat. Once your wealth increases to the Nisab level, you would begin counting 12 lunar months from that point onwards.