I think it just depends. Last year I was able to book tickets to Barbados online with USAir about 10 months out or so. I have also had good luck with Delta with much shorter-term bookings. Just last week I booked two tickets on Delta to St. Maarten for this July, and last September booked first class to Palm Springs for next month. All of these were done online, and all flights were on weekends. I have also had good luck to Europe with several FF programs, but in those cases have had to call in as the itineraries were more complicated (stop-overs).
I do think that some seats may open up later on (depending on the airline), but obviously the earlier you try the better.