When answering this question, it is important to remember that most actions taken by an airline relate to their ability to generate profits.
Passenger airlines generally publish future schedules so that they can sell future tickets. It is therefore in their best interest to publish schedules that they intend to fly, or they'll have to issue refunds for any tickets they have sold.
For the most part, airlines don’t really focus on their schedules outside 6 months – they are basically extensions of the closer-in schedules with known seasonal adjustments. Sometime between 4-6 months, they figure out their levels of service. Once inside 4 months, they’ll lock down equipment type, and the time of day. Inside 2 months, most of the change is due to operational issues (crew, maintenance, facilities). Once the schedule has the crew assigned (about a month out for the start of the crew month), something major has to cause a change – e.g., environmental issue (hurricane, SARS, volcano, etc.).