If there is a potential to use frequent flyer miles, do it at 331 days (depending on the airline). If you are purchasing the tickets, it's tough.
For our trip last month, our two older kids were making their own arrangements. All of the special fares expired the day we were leaving home (summers are more expensive). I signed up for every airlines' email updates and started reading flyertalk.com . I saw a post on that site regarding flights from Atlanta (about 200 miles from us) to Honolulu for $354 roundtrip. My daughter ask if they would be any cheaper and I urged her to book immediately, which she did. My son waited until four or five months out to decide he wanted to go and the cheapest fares were around $700. Then he couldn't even get the dates he wanted without paying lots more than that. He wound up paying a lot, then was forced to go five days later than the rest of us and come back two days early even to get what he got. A couple of weeks before we left, I got an email showing available flights for about half what he paid. You just can't win.
We are planning a trip to the British Isles next June and I don't even know how to start to find the best fare.
Sheila