I'll make a deal. I'll tell you exactly when to buy the cheapest airfare if you tell me what tomorrow's winning powerball numbers will be.
No matter WHAT anyone says, there is no "best time" to buy airline tickets. Just for fun (and my knowledge), I'm doing an experiment. Every week, I'm checking prices from my home airport to three different destinations. I'm checking on Tuesday or Wednesday (supposedly the "best day to book") and using Travelocity which shows me multiple airlines. Not exactly a scientific experiment, but at least has some data.
We are only served by four airlines... Delta, US Air, United, & American. No JetBlue, no Southwest.
I started tracking at 318 days from departure (that would put it 325 days from return, and bookings open at 330 days). So far, the CHEAPEST rates have been the first three weeks for ALL THREE destinations.
There have been some momentary dips in cost, but still not down to what the cost was those first three weeks. This includes prices at the "6 month mark". I haven't gotten to the "three month" yet.
The bottom line is to book when you're comfortable with the cost, and don't look back (unless you're flying an airline that provides price drop credits).