AC has announced definite expiration dates for eUpgrades on their Altitude 2015 page:
eUpgrade Credits earned on or after November 7, 2014 will be valid until February 29, 2016. Additionally, eUpgrade Credits earned on or after November 1, 2015 will be valid until February 28, 2017.
I'll give a ^ to AC, not for the expiration dates themselves (since I would still think 6 months should be the minimum for credits), but for finally taking the guesswork out of it and also announcing next year's schedule.