In preparation for a trip to Amsterdam and return from Budapest for this Oct, I google best time to purchase international and was surprised to find times at approx. 270 days for Europe and 330 for Asia. I was way past the 270 for our Oct trip so ended with taking flights on BA which were still a 1-2k less than anyone else for one stop travel.
We have a similar (headed to Rome instead of AMS) trip in March 2018 and fares nearly 1.5-2.0 times what I was seeing for this Oct.
It seems the airlines really want to push you into a two stop itinerary if you want to save any bucks.
Nwsteve
Nwsteve - my research a few years ago indicated that just under 60 days for international is the best time to purchase, and that has been confirmed by my personal experience. If a great airfare sale shows up earlier, as happened last year (half price!!!), I'll book then. Otherwise, I am confident that I can wait until 60 days for prices to drop, and my experience is that they don't start going up again until a few days after the 60 day mark, and slowly at first.
I confess, however, that I rarely look at a flight more than 6 months in advance. And frankly, I don't make my plans so far out. But according to my research a while back I didn't need to book months in advance.
If you are trying to use reward miles to pay for a flight, then yes, you do need to book well in advance.
I'm also often buying tickets from US to Amsterdam. Sometimes open-jaw, sometimes not. I bought my current RT tickets to Amsterdam around 60 days out, and just after 60 days they actually dropped $10. I started watching at 120 days out.
One of the biggest determinants for airfare prices in Europe is whether the flight is in high season or not. High season starts late in May, and goes to something like August 23. If your outbound or return flight is outside of that window you can often find tickets a few $100s cheaper.