Stop and think about your question. If there was a known magic time to buy the cheapest ticket, then the smart people would simply wait till that time. There is no best, magic, special, cheapest time to buy airfare. It is like trying to buy a stock at the low point or sell at the high point. No one knows. If they say they do, they are just blowing smoke and you are a sucker for believing them.
Our experience, and it is just our experience, is that the consistently cheapest prices are about nine months out. The prices climb slowly with an occasion dip of $50 to 100 for a couple days. Sometimes, but it seems to be less frequent these days, there can be a big drop for a short time around the three month mark. But on our last trip that never occurred. Remember all the ticket prices are being set by computer programs that are constantly monitoring the sales. And those prices can literally change hour to hour and almost always day to day. It is game that is difficult for the average consumer to win because you don't know all the rules.
If you have been watching the prices for a while you should know the range. If it hits the low end of that range, then buy and don't look back. You can always gamble that the prices will be cheaper in the future but what if they are not. The other drawback to waiting is that some schedules might not be available and seating becomes limited.