You can monitor prices of similar flights before the actual flights you are looking for are made available. Thus you can get an idea exactly how the airline operates it's fare offerings. It used to be the best fares were at initial time of release, but now its much more common for fares at date of release to be high, then drop at some point after release...10 days, 2 weeks, 30 days, 60 days. Every airline is different. If the OP is specifically looking at United for their IAD-BCN non-stop, the lowest fares will start 15-30 days after initial release. But if you don't do your research, you won't know when that is. And there isn't some magical date, each and every time. Again, you have do research, and monitor regularly. There are some programs/websites out there that can monitor for you.
This is critically important if you are doing award seats.