What is the optimal time before departure to buy the cheapest long haul airline ticket?

What is the optimal time before departure to buy the cheapest long haul airline ticket?

By the ticket early

There's no mythical optimal time. There are too many variables that determine the price of a ticket.

No magic formula, although you should be looking to buy 3 months ahead, or factor on sales to region quieter in off season. For bush peak period locations, it maybe better to book. 6 months ahead or as soon as travel plans are firmed up.

The optimal time is when the fare is cheapest.
There's no worldwide day for the 100,000 daily flights. For cheapest price ever on a flight.

In most cases you get best price the earlier you book. BE WARNED the very cheap price is usually NO REFUND. Some sort of travel/trip interruption insurance is a good idea.
Travel in LOW season if you want CHEAP. Avoid weekends if you want CHEAP look on low cost Airlines for the CHEAP fares.
Only you know your value of time. The search sites look for the FASTEST connections not every possible way from one place to the next. A different route a few hours extra wait between flights can give a much lower price,
NO WEBSITE checks for the millions of possible interconnections on the daily 100,000 flights for the absolute cheapest fare possible. They look for the fastest. ONLY YOU can give a value on your time for the hour or day longer plan from departure place to final destination.

The low cost airlines are not listed on most search sites you look them up one at a time to see if they have something for you.

If there was one magic day to book the entire world would know and websites would crash from overloading of booking requests.

There's no fixed rule since the answer may depend on various factors such as travel in peak (e.g. School holidays) or low season, competitive route or not (compare New york to London versus Paris to Kinshasa), only direct flight or do you agree with connecting flights, and so on.

However in many cases, as for long haul flights, the cheapest deals are available 5 to 6 months before departure.

Read more:

and about season in air travel market:

BTW
You may save also a lot of money when you choose the cheapest travel days (e.g. Midweek flights versus weekend flights). So, always check airfares of different travel days if your travel planning is not fixed.
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As soon as you're 100% sure on your travel plans. The least expensive tickets always have the moat restrictions like NO refunds, minimum $200 fee to change dates PLUS difference in airfare and limited checked luggage and seat selection. If all of that is cool with you, 6 to 9 months out on most routes.
Keep in mind that most long hauls run once a day which means once the seats start running out, the price only goes up. You have more flexibility on a route like new York to London because there are dozens of flights a day. On a route like Austin to London, there's only one flight a day so prices will just go up

Between 3 and 5 months before departure.

It depends on when you want to travel. If you are wanting to fly during peak season, you want to start looking early. Otherwise 3-5 months would work. My recommendation is to start the search early. Once I bought a ticket for my wife and son to fly to Asia during the summer but bought it 10-11 months before their departure date. The ticket price was really good and when I would go to check back on the price, it was never as low as what I paid for it. I think it is not always easy to get the cheapest ticket so my suggestion is to set a budget and tell yourself approximately how much you are willing or hope to pay for this ticket. Then just keep looking. Some website may alert you when you tell them of plane tickets that reach a certain price.

Despite what a lot of people say, there's no way of knowing. If there was an optimal time it would have become known and everyone would buy their tickets at that time, which would stop it from being the optimal time.
You take your chances like everyone else.

3 months before departure start looking. Can you be flexible? To 2 weeks before departure. Sometimes you can add with in 3 days of date. Or departing can you pick up a free flight to Vegas. Then fly out of there to Asia? Some time s you can catch gambling junkets with a seat or 2 open for cheap.