I did a trip to Ireland 3.5 months after my first THR. It was mostly driving, with small walks and short day hikes. But there was one day in Belfast where I must have put on some good mileage. The ironic thing, though, was that my new hip bothered me not at all. It was the other 'real' hip that was totally angry with me. The key was that I didn't plan - and didn't do - long walks every day. It just happened that we were having a good time, and I never felt like I was overdoing it. That is, until the other hip decided to chime in.
All depends on the type of trip, and how active you want to be. I even found the flight to not really be an issue. Just got up every so often, moved a little, and relaxed when I could.
Where in Europe are you guys planning on going? So many places there I still want to see. I think Germany is next for us when we go.