See the Top Question (upper right corner of the page... "Buy now or later? What's with these screwy ticket prices?" That should give you some information.
Ultimately, while we know tickets get more expensive as the cheaper ticket fares are purchased, when that happens is unknowable. Certainly airlines have pre-purchase requirements for some of the cheaper fares that trigger 21 and 7 days out from departure. Other than that, it depends on demand.
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