For air travel, the magic number is 827 August 2010
Economists have calculated that the optimum time to book an airline ticket is eight weeks before the travel date. Also, do it in the afternoon rather than the morning to get the best deal.
Book earlier than eight weeks and you run the risk that your personal plans may change and the booking may need to be altered, but leave it later and there’s an increased risk that prices will go up or the flight could be fully booked.
The work by Japan-based economists Makoto Watanabe and Marc Moller has been published by The Royal Economic Society.
They also highlight differences in timing policy in booking flights compared to booking theatre tickets, where last-minute deals can be available.
source: The Australian