I believe you, but on the website T&C I searched "expir" and all I saw was the confusingly worded information about the account, not actual miles:

If a Blue or Gold Member has not flown for at least once within a period of thirty six (36) months from the date of his/her last registered and Account-validated flight, then his/her Card and Account will be automatically inactivated and cannot be used for any purpose (mileage accrual or redemption) for a period of 12 months. During this period the Member may retroactively claim a flight he/she had made within the previous year so that to have his/her Card and Account reactivated. Otherwise, as soon as the 12-month period expires, the Card and the Account will be irrevocably canceled.

The miles collected by converting points from the Aegean Bonus Visa card are Award Miles, as defined in chapter 3 “MILES/CATEGORIES ETC.", and never expire.