In his answer to this question Dr. Monzer Kahf, Scholar in Islamic Economics & Financial Expert, who stated:

Individual payers are required to give away their due zakah as soon as it is due on them, at the end of the lunar Zakah year. They can make the same during the year payment, i.e., start paying as needs arise and then at the due date make calculation and pay the balance.
But if one gives one’s Zakah to an Islamic organization and it carries a program of distribution through time, it is ok for it. Of course, it is always better to avoid leaving any balance to be carried for next year, unless it has plans to build long term assets for the poor and needy.