The miles you earned in any particular month will expire three years later at the end of the same month. Not a lot of time.

From EVA

http://www.evaair.com/html/b2c/engli...e_Validity.htm

Mileage earned by traveling or partners will be retained for 36 months from travel date or partner's system apply date. For example, miles credited for flight date Sep.15, 2010 will expire on Sep.30, 2013. Earned mileage within its validity is redeemable. The expired mileage will be automatically cancelled without notice.