Alaska Mileage Plan miles do not expire as long as your account shows some activity at least once every 24 months. Activity can be something small, like earning a few miles from a flight, using a partner hotel or car rental, or even buying a small amount of miles. If there’s no activity for two full years, Alaska may close the account and the miles can be forfeited. It’s pretty easy to keep the account active, though — even a single earning or redemption action resets the 24-month clock. If you’re unsure about your status, you can check your activity log online.