If you’re looking for the best possible time to buy the cheapest plane ticket, try 54 days from your day of departure.

That is the exact moment when you will save the most on domestic flight tickets, according to a study by CheapAir.com that analyzed 1.3 billion airfares over the last year.

If you’re looking for a period of time in which you are hoping to get the best savings on your airfare, CheapAir.com has called the window between 21 and 112 days before departure the “Prime Booking Window,” where prices are at their lowest.

The findings also dispel with the notion that airlines dramatically lower their prices the closer it gets to the departure date in an effort to occupy their planes.

“Generally, a trip price starts off high, slowly drops and then starts to climb a few weeks before the flight,” said Jeff Klee, CEO of CheapAir.com. “People ask all the time if it’s true that at the last minute the airlines have unsold seats that they practically ‘give away’, but that’s rarely the case. Fares usually go up dramatically within 14 days of the flight.”

To help you game airline prices, you can think of the best periods to purchase tickets as specific blocks of time. Most airlines start selling tickets around 335 days before a flight, Klee said, but anyone buying a ticket almost a year in advance will probably shell out around $50 more than they would during the “Prime Booking Window.”

From 113 to 196 days out, travelers will have more flight route options to choose from, and tickets are just $20 more than the “Prime Booking Window.” Bargain hunters will want to wait for the prime discount window for the best deal, and fares will tend to fluctuate day by day during this period.

However, once you’re around two weeks away from your flight, travelers pay an average of $75 over the cheapest fares, and from one to six days in advance, that premium shoots up to $200.

“54 days is a good number to start with, but it’s important to know that every trip is different,” Klee said.