Leading economists have suggested the cheapest time to buy a flight is eight weeks before you plan to travel, but a top travel boss disagrees.
Marc Moller and Makoto Watanabe looked at why flights get more expensive as the date of travel gets closer, in a paper published by the Economic Journal this month.
According to the research, airlines opt for advance purchase discounts far in advance of travel rather than clearance sales at the last minute, as the capacity on flights is limited.
When purchasing early, consumers are uncertain about their plans for the future, so risk the possibility of unforeseen events changing travel plans. They therefore need an incentive to book early - hence flights are cheaper further in advance.
However, the researchers suggest that flights are cheapest eight weeks before the planned date of travel.
The economists also said that flights may be more expensive in the morning than in the afternoon, as business travellers book their flights in the morning from work.
Stewart Harvey, HRG's commercial director, disagreed with the economists' findings in terms of business travel.
He suggests the best time to book flights is fifteen working days before the date you want to travel.
"For a large number of corporates, business life changes so frequently that anything more than a month in advance is subject to a very high percentage of change," he said.
Harvey also recommends not booking the lowest fare, as this often has restrictive conditions.
"Take a lower fare, but don't take the lowest. Take a fare that will you give you a cancellation or amendment fee that still has the ability to put the remainder value of the ticket towards a new one."
Harvey said there is a "sweet spot" to aim for, although difficult to find.
"It gets the right balance between cash flow, your cancellations, your business behaviour, giving you a bit of discipline, but doesn't mean that you are held to ransom by restrictive terms and conditions."
Predicting air fares on any given day can be a guessing game, according to Harvey, as airlines yield management programmes have become so sophisticated.
"Over the last couple of years they [the airlines] have become a lot sharper in the way they manage the yield," said Harvey. "Trying to predict it is so difficult."
Airlines look at demand and traffic flow not just by route and day of the week, but by the time of day.
"That shouldn't surprise us," said Harvey, "because if you are a commuter into London, any train in before 9.30am is going to hit a peak-time fare. Airlines do the same. They think the same way. They've just got it down to such a variety of fare levels and combinations of fares, they can vary it to help their demand by time of day."
Early morning flights on busy business routes are more expensive, while flights in the middle of the day are cheaper.
In terms of flight booking times, however, Harvey disagrees with the economists' suggestion that flights are cheaper to buy in the afternoons, as business travellers book in the morning from offices.
"Traditional nine to five thinking went out of the window for us years ago." Business travellers often use self booking tools, or mobile devices while on the move to plan their trips, rather than from the office, said Harvey.