I think the general suggestion that I see floating around is 6 to 8 weeks out. I, however, don't follow that guideline. I fly from a smaller airport where I find that flights tend to only rise from they are posted. Once I find a good flight for a price I'm comfortable with, I buy.
If you fly from a bigger airport, or a 'hub' airport you may find that you can get really good deals about 6-8 weeks out directly from the airline. I'm a JetBlue True Blue member and I get emails all the time for really cheap deals (my airport usually isn't included.)