Offers expire after 12 months. This is done in order to keep content fresh and encourage updates to be made seasonally (including price, image, description, etc.).
You will be sent a notification that your Offer is about to expire 7 days and 2 days in advance of the expiry. You will also be sent an email on the day that your offer has been expired in ATIS.
When an Offer has expired, it remains in that state until it has been revived by a user (by entering new current/future dates). An Offer is only temporarily unpublished or permanently deleted when a user has requested it.