Shopee Coins will expire at the end of the 3rd month after they are credited to you (e.g. Coins earned anytime in June will expire on the 30th of September). You may check your Coin Balance and expiry date under your Me page.
When use Shopee coins to offset the price of your purchases, the oldest coins will be spent first.
Go to the Coins Rewards page and tap on your Coins Balance to see your Coins history and expiry dates.