A study from CheapAir.com aims to answer that instrumental trip-planning question: How far in advance should I book my flight?

Number crunchers from CheapAir.com, an airfare booking site, monitored 4,191,533 trips last year to find a solution. They looked at fare fluctuations from 320 days to one day in advance of departure to determine how far ahead of time travelers should book plane tickets. All in all, the folks at CheapAir.com analyzed the booking data from roughly 1.3 billion airfares.

According to the results of the study, the best time to book a domestic flight is 54 days (or seven and a half weeks) in advance. For international flights, CheapAir.com generally advises travelers to buy earlier rather than later, but prime booking dates vary by destination.

The worst time to get that ticket? At the last minute, as you would expect. The lousiest airfares were booked a day before departure, the second lousiest airfares were snagged two days before departure, and so on—up to 13 days before takeoff. According to CheapAir.com, "Buying a flight with less than two weeks' advance purchase is the last strategy we would recommend."

The reverse case yields overpriced tickets too. Book too early and you're missing out on the price slashing that happens as airlines attempt to fill empty seats on unsold flights. Here's what CheapAir.com has to say about it:

"According to the data, sometime around 225 days out (seven and a half months), on average, fares started to drop and by 104 days out (three and a half months) they had fallen to within $10 of their low point. From there they continued to drop, slowly but steadily, until reaching a low 54 days before departure. After 54 days, fares started to climb again, remaining within $10 of that low until 29 days out. Then, the increase began to accelerate and once you were within 14 days the fares really shot up dramatically."  

The takeaway here isn't a hard-and-fast solution to your grueling airfare search. The smartest thing you can do is keep that prime booking window in mind when browsing fares. Don't book too early. Don't book late. And ramp up your search efforts about 100 days in advance of your trip. These guidelines coincide with what we've advised in the past; broadly, there's a best time to book if you want to snap up discounted tickets before they run out: between three to four months and one month before departure.

There's one big exception to these ground rules, though. When traveling on highly popular routes, such as a flight to Orlando during spring break or flights around Christmas or Thanksgiving, it's definitely best to book early rather than late. Airlines won't slash prices on mostly sold-out flights, and you'll want to grab the best seats as soon as you can—especially if you're hoping to sit next to friends or family. The data from CheapAir.com showed that the best time to book flights for Christmas travel is six months before departure. 

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