بسم الله الرحمن الرحيم
The amount which you have fixed its zakah is wajib on you. If you pay it each year then it is better but if you pay the zakah of all the previous years together when you receive the amount then it is also better.
It is not lawful to have fixed deposits in banks as it is interest based dealing; the extra amount which you will get from original amount shall be interest and it is wajib to give it in charity without an intention of reward.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.