21 days

Fliers looking for the best prices should book exactly 21 days prior to departure. Book in that sweet spot, Kayak says, and you'll pay the domestic average of $342 a ticket -- versus the $370 you'd pay on average for a ticket booked six months out. For international trips, that prime booking window shifts to 34 days prior to departure, with average prices at $977 instead of $1,016 for travelers planning six months ahead. The number crunching also proved what many frequent fliers already know: Those who wait until the last minute will almost certainly pay more for their fare.