According to flight data from 2013:
Fifty-four days before takeoff is, on average, when domestic airline tickets are at their absolute lowest price. And if you don’t hit 54 days on the head, you should usually book between 104 to 29 days before your trip — within the “prime booking window” — for the lowest possible prices. In this window, ticket prices typically hover within $10 of the lowest price they’ll ever reach.
Also, never book 2 weeks before your flight. That’s the worst time ever. Read more here.