|Quoting Nkops (Reply 1):|
I believe a lot of airlines do 11 months in advance, however it is subject to change multiple times before that time. WN does 5 months I believe (it is on their website). Probably different for each airline.
I believe it is different for each airline. Some airlines prefer to do it earlier, others like to see what the competitive landscape looks like before publishing their schedules (and even then, it is subject to some change).
An airline wouldn't want to leave it too late, however, as they might have a harder time selling the flights.