Rewards typically expire after 30 days. However, some companies may set an expiration date that is shorter than 30 days. The expiration date will never be sooner than 3 days after the reward was originally sent. You can see the expiration date for your reward at the bottom of the email letting you know you've been given a reward!
Once the Reward expires, you will not be able to claim the Reward unless your employer sends you a new Reward.