All Post box renewal should happen on the 1st Jan of any new year. The validity of any Post Box contract is Jan - Dec regardless of when you initially applied to open the Post Box.
If you do not renew - late payment fees come into effect on 1st February. Postponing the renewal for many months comes with the risk that Emirates Post Office can reassign the Box to any new Company and this Post Box Number would be mentioned on your Company Trade Licence amongst other documentation. Some of your staff will use for delivery of their Emirates ID Card so please be aware it is important to maintain the validity of the Post Box Number.
Also a gentle reminder to empty your boxes regularly, if left unattended for a length of time the Post Box will fill up to capacity this will lead Emirates Post Office to close and lock the box and there is a fee to unlock.