This has come up on here a few times recently, generally 330 days, there was a report recently of call center being able to see a big further ahead. You can book with the latest possible date now, then call them to request them to add a note for a free date change since the desired return date is not yet available. If you're worried they won't do this you could also book on the phone and have them to this immediately, or call them first to confirm it before booking online.