Most airlines sell seats up to 350 days in advance, so you can book on a GDS for jan 2012 now.
But schedule changes can happen, mainly at the season change in March and Oct, and changes not finalised until a few weeks before the season change.
Any pax booked will be rebooked or re-accommodated where needed.
Situation is different for those who only sell online - ie Ryanair. They tend to release schedules 6 months in advance.
No hard and fast rules for all carriers 0 these are just general guidelines.