The simple answer is as early as possible.

We at Future.Travel work with passengers from all over the world. A lot of them are expats who live in one country and fly ‘home’ 1–2 times a year, sometimes with the whole family. The common denominator for most is they are booking 9–12 months early. Most all take out travel insurance. They pay about 50% of the normal price because of good shopping, and early payment. We are constantly amazed at how cheap airfares are that they find. They don’t book discount airlines generally those LCC airlines don’t tend to release more than a few months supply of seats at time, particularly on key routes.