Berlin to Istanbul is about 1,685 km and usually takes around 3h in the air. The track it follows runs over Central Europe, the Balkans, and the Eastern European plains.
Over Central Europe, the Balkans, and the Eastern European plains 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.