How far ahead can you purchase a plane ticket?

i usually buy plane ticket about 3 mos early. i wonder if can be earlier? when iknow im planning on vacation i usually by the tickets early because they are cheaper.How far ahead can you purchase a plane ticket?currently if you go on expedia, you can purchase tickets that depart in march 2009 at the latest--- so about a year. basically if you want the best prices buy your plane tickets the moment you plan when your trip is going to be; it will save you the hassle (and money) in the long run.