IT does really depend. What I do, is I always have an idea on what I'd like to pay, and then look for that fare. I usually don't book my tickets no farther than 6 months out. Also, take into consideration, by booking airfare VERY far out, you run a high risk of serveral schedule changes between now and your flight. It can wreck havoc on your nerves. As previous posters stated, get an idea on what you want to pay, and if you see it, jump on it. Most airlines don't offer their cheapest seats until around 3 months in advance. If you aren't traveling during a specific busy time, ie holidays, you usually can wait until then. Also, keep checking how full the planes are, you can easily do this in Expedia and not have to purchase a ticket to see the seat map.