Istanbul to Amsterdam is about 2,183 km and usually takes around 3h 30m in the air. The track it follows runs over the Balkans, Central Europe, and the North Sea.
Over the North Sea, the Balkans, and Central Europe the air is usually calm and settled. This track threads jet-stream latitudes, so in winter you may feel a few minutes of light, choppy air high up; summer usually rides calmer.
For most of a flight this length you are at a steady cruise above the weather, where the ride is smooth; the livelier moments, if any, are short.