At least until someone else comes up with a study.
http://viewfinder.expedia.com/img/STOR-23513_White_paper.pdf
For international flights, the study concluded that the lowest price is 171 daysout (about six months), which pretty much throws water on the two-month"sweet spot" theorytoutedpreviously by someone here.
But if you look at the graphs, prices are fairly consistent from 300 days out to almost 100 days, with a slight downward trend from 300 to 171, then easing back up to the 100-day mark. After that the price takes off---upward, steeply. There is a tiny little blip of a dip right before the day of the flight, but it is nowhere near the lows six months earlier.
What I found more interesting is that they confirmed what I have observed with my own flight research---it is cheaper to fly to Europe on Thursday and return on a Monday. One can save as much as 20% over travel on Friday, Saturday, and Sunday. For most of us that means taking an extra three days off work---not always possible.
I didn't read all of it so don't know what else it says.