If you are looking for a simple answer, I’m sorry but it doesn’t exist. A lot of different factors can change the price of flights (inventory, sales, and competition). Fortunately, there are some general trends that have recently been found from a study conducted by Expedia. Here are the results:
Domestic Flights: book 50-100 days in advance. Booking prior to 100 days can actually be more expensive.
International Flights: the best deals were found at an average of 171 days prior to travel, although flights were still relatively cheap up to 50 days prior to travel.
The best advice that can be offered is to check the flights frequently to get the best deal. Or an easier approach would be to sign up for a price alert on whatever booking site you prefer to use.
www.airfarewatchdog.com is my favorite resource for this!