I'm not a really frequent flyer, but I do have a few longhauls under my belt. All of them were transatlantic with the longest being 9 hours and 13 minutes from ZRH to ATL on a Swissair A330. PEK-CDG will be a good deal longer than that, but I'll tell you what I can. Try to sleep. If you cannot sleep on an airplane, then try to get your doctor to prescribe you sleeping medicine. I would recommend getting them to prescribe you somthing that will really put you to sleep, not something like Ambien that just gets the chemical balance rigth to sleep, but doesn't actually make you tired. If your doctor won't give you that, then take two Benadryls. Those are allergy medicines, but they are frequently used in hospitals (so they are safe for this) for their side effect, drowsiness, and they really do make you drowsy, especially two. Air France has PTVs on their 777s, A330s, and A340s, so if the route is flown by one of those, then you can enjoy that. If you have a window seat, then that will take up a lot of time looking out. Bring books and music. Talk to the flight attendants. Savor every meal, not matter what it tastes like. My first longhauls (ATL-BRU, BRU-ATL) seemed really long, but now I love longhauls. Have a great time!