How far ahead to buy plane tickets?

Always buy a round-trip ticket (or an open jaw, where you're flying home from a different city - but always have your flight home from Europe booked before you get there). UK passport control officers will ask you how long you plan to stay in the country, and they may ask for evidence of your onward travel plans. If you don't have any, they might not let you into the country.

There is no magic time to buy plane tickets. Start monitoring fares as soon as you can (you can usually check fares up to a year in advance), and buy when you see one that looks good to you.

I start watching them a year ahead of time. Plug in your dates as if you were going this year and see what price they are, then keep watching every few weeks to see how they change. If you track them then you will get a good idea of what a good price is. I generally buy about 6 months out. I've waited and watched them climb, but you just never know. When you see a price that is acceptable to you, then buy. Tracking them will educate you.

If you will be traveling elsewhere then buy open jaw or multi- city, into one city and out of another. If you are staying put then I'd buy RT. It used to be that one way tickets were not that much less than RT, but today some airlines offer one way tickets at reasonable prices like Norweigen and Wow airlines. But a normal US carrier like United or American you do better RT. There are many website to help you search like kayak.com etc. You can also set up alerts so you get an email about price changes.

From our experience we generally find the best price about six months out or the two weeks before. Remember you can only stay in the Schengen zone for 90 days so you need have your return trip finalized.

I usually buy our tickets 8-9 months ahead of time when I feel like it's an acceptable price. Buying them early gives you the chance for better seats, I.e. The first row of Economy Plus.

I bought my tickets today for a trip in late Sept. 2016, which is five months out. I have been watching fares for the last three months and I saw my multi-city itinerary was at what I had decided I wanted to pay I pounced on it. I used Kayak but I know the others work the same way. Good luck.
Cynthia