You simply cant generalize. I will say that for the most part airline fares do increase as the day of departure nears. Airlines do periodically have last minute (read: starting about 3 or 5 days prior to departure) offers on some routes that are low performers. But, my experience is that its better buy earlier (but not too early) rather than later.

I'm actually doing a mileage run next week. My round trip fare to LAX is $115 (lowest that Ive ever found). The same routing today is $320.