I think, depending on the airline, you can book up to 10 months in advance, from what I've seen. But if you do decide to book so far in advance, you should definitely re-confirm closer to the time.
Note that this doesn't necessarily mean you will get the cheapest price, often airlines will cut prices closer to the time to fill up the flight. I know that SAA do this, I am sure that the other airlines will as well.
B