From Yahoo finance.
Bargain-conscious travelers have been trying to answer the question for
years and are still stymied: How far in advance do you have to book to
get the best airfare?
According to new research by CheapAir.com
based on the travel site's review of 560 million airfares, the optimal
time to book a domestic flight is 49 days in advance. If you're flying
overseas, you should book almost three months -- 81 days, to be precise
-- before you travel.
Read more by clicking on this link.