It goes without saying, but here it is nonetheless: If you're traveling tomorrow and you start looking today, your chances of finding a great deal decrease rather dramatically.
The earlier you start looking, the better. Most airlines recommend buying tickets at least two weeks in advance to get their lowest fares, and some extend that to three weeks [source: Hobica]. Airlines increase prices as the travel date nears, reportedly to take advantage of typically short-notice, inflexible business-related travel [source: Kayak].
It's possible to find last-minute deals, notably through "name your own price" Web sites, but many travelers are uncomfortable shopping that way because you don't know exactly what you're buying until after you buy it. (Yikes.)
So, ideally, you'll start looking with weeks to spare. But that's a minimum. To pay rock-bottom prices, it's best to start checking fares months in advance.
And not just once a week.