For the best deals, Kayak’s 2013 data shows you're going to have to book way earlier if you're heading to South America or Asia. Though I usually adhere to the one-season rule, in the name of science, I’m willing to defer to hardcore facts.
Kayak recently ran a study on their 2013 numbers and compiled some general tips all in the name of cheap airfare. The data is based on the millions (billion?) of queries they received throughout the year and sheds some interesting light about the correlation between your destination and how far in advance you should buy a ticket.
I’m a big advocate of beginning to shop for your ticket about four months out, with three to two months beforehand being about the time when you book your ticket. The data seems to generally hold true for this theory, except for Asia, South America and the Caribbean. In particular, you should start shopping almost a year out for the best deals on Asia; South American adventurers need to start looking about half a year out.
The Caribbean seems to be one of those places you can go last-minute, even up to a week out. Dudes, it’s not too late to put together something last minute for this weekend. Your girlfriend never has to know.
According to Kayak, the rundown looks something like this:
-Domestic: Book between 3-7 weeks out
-Europe: 8-10 weeks out
-Asia: 9-10 months out. If you miss this window, shoot for 3-4 months out
-South America: 6 months out
-Central America: 6-8 weeks out
-Africa: 3-5 weeks out
-Caribbean: 2-3 weeks out, but unlike all other regions, you can still get a deal one week out (i.e. it's a great last-minute destination)
In a couple of other interesting tidbits, it also doesn’t seem to matter what day you book your ticket on. I’m still convinced that the best day to buy a ticket is on Tuesday, but that’s just my own personal superstition. (You can go ahead and break that mirror if you like; I’m good.)
There’s also some data on the best days to travel, but for the most part, those results aren’t too surprising.