Casablanca to Istanbul is about 3,350 km and usually takes around 5h in the air. The track it follows runs over North Africa, the Mediterranean, the Iberian Peninsula, and the Balkans.
Over the Mediterranean, the Iberian Peninsula, and the Balkans the air is usually calm and settled. Above North Africa daytime heat can stir shallow thermals lower down, but the air smooths out at cruise. 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.