There are too many variables in the equation. For example, I could tell you how many days before departure to buy a SEA-PHX r/t ticket, based on historical data. What that doesn't tell you is that DL just started 5 new flights per day on the route. DL may offer a fare sale to stimulate demand. Other carriers may have excess seats available, generating more low fare availability closer to departure. Other carriers may offer fare sales and/or bonus FF miles that generate additional demand.

And, that's an example where the new competition is very clear. If we were talking about PHX-SYD, we might be looking at an airline offering an additional flight out of LAX (which might have an effect on fares, schedules, etc.). So, as we know, the airline situation is very fluid. The best thing is to know your routes and your fares. That way, if a sale comes up, you know whether is it really low or not.