As per Eva Air website, the miles you earn in this membership program are valid for 36 months from the travel date. The expired mileage is automatically canceled without notice. If three years are not enough for you, you may consider other ways of earning rewards, such as getting a travel credit card. There are cards on which accumulated miles never expire. For example, miles don’t expire on the Discover it® Miles, and you can also redeem them for any airline, not only Eva Air.