Istanbul to Casablanca is about 3,350 km and usually takes around 5h in the air. The track it follows runs over the Balkans, the Mediterranean, the Iberian Peninsula, and North Africa.
Over the Mediterranean, the Balkans, and the Iberian Peninsula 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.