The airfare shopping site CheapAir.com monitored more than 4 million fares in 2013 to answer the age-old question: How far in advance should you book your plane ticket?
For the average domestic flight, booking 54 days in advance will offer the cheapest fare.
But CheapAir's CEO Jeff Klee told Business Insider that travelers should worry more about a window of time than an exact number of days. When the site studied the cost of a flight everywhere from 320 days to one day before departure, it found the prime window for purchasing a ticket sits between 29 and 104 days before takeoff. 
"You should monitor prices actively in that period to get a sense of the market," Klee said. "When you see a good fare, grab it that day."
If the airfare drops even further in that prime window, CheapAir maintains a policy to refund travelers up to $100. He said airlines have designated between 15 and 20 different fares for the same seat on a flight, based on a variety of factors.
"The single biggest factor in a price increase is how full a flight is. You should also consider how many flights travel the route you're looking for," Klee added. "People are surprised how little mileage has to do with the calculation of airfare."
"The worst thing is to book a ticket too late, but the next worst thing is to book too early when multiple routes have yet to be planned, and you have fewer options to fly somewhere," he said.
At some points in the year, flying from Los Angeles to San Francisco costs more than flying from Los Angeles to London because fewer flights are offered on the first route. The fewer available options to fly to a certain place, the more expensive it will be to take that flight. Also, the fuller a plane gets, the more expensive the seats will become, Klee said.
For international flights, you'll want to purchase tickets even farther in advance. Travelers going to Africa should book the earliest, at 166 days in advance, while those heading to the South Pacific can book much closer to their departure, at an average of 70 days in advance.
