Do my Stars or free Rewards expire?
You can redeem for a free Reward each time you collect 125 Stars as a Gold member. However, Stars expire six months after they are earned on the first day of the month after a full six months.
For example, if you earn 10 Stars on June 16 and 15 Stars on June 24, your total Star balance for the month—25 Stars—will expire on January 1.
You can track upcoming Star expiration in History in your account online or in your Starbucks app for iPhone®, Android™ or Windows phone.
When you redeem Stars for a Reward, Starbucks will apply the oldest Stars in your Star balance toward your Reward.
See here for Birthday Reward expiration.