Airlines book up to 1 year ahead, depending on the airline. Discount airlines may not actually confirm pricing for some routes until 90–120 days prior to departure.
We see at Future.Travel that some carriers will not release pricing or inventory for high demand periods (Christmas, New Years, Lunar New Year) where they know they have a captive audience until the ‘last’ minute (read 60–120 days) before the holiday as they know they will fill the plane. The thinking is to get the higher revenue passengers who appear closer to the travel date.
On the other hand, some airlines like to have cash in the bank for 11 months to offset the lower cost fares they offer for an early purchase. Our best buyers are expats and families who buy in January/Feb/March for the following Christmas and New Years family travels.
Beyond 12 months… it is too far out for scheduling of flights and pricing as it will fall into a revenue cycle that has not been determined for fuel hedging, airport landing fees, and local/national taxes.