You have to use your reward points within 1 year of accumulation. Points will expire in the order shape which they have been first earned.
For example, if X reward points are accumulated on the 1st of December 2019 and Y reward points are accumulated on the 1st of January 2020 then X reward points will expire on the 1st of December 2020, and Y reward points will expire on the 1st of January 2021.