Different sources will give you different timelines.
I start searching as soon as I have my dates set.
I set up fare alerts on sites like Kayak and Orbitz.
Also check sites like ITA Matrix or Vayama.
But I also do random searches several times a week and at odd hours.
The best fare I ever found was on a Saturday night 6 months before our trip.
Just happened to catch a 12 hour unadvertised sale.
Follow price trends and be realistic in your expectations.
Pick a number you like and when you get it or close to it, buy.
Most importantly, don't stress. You're buying tickets to paradise!