Karachi to Istanbul is about 3,915 km and usually takes around 6h in the air. The track it follows runs over the Middle East and the Anatolian plateau.
Air lifting over the Anatolian plateau can raise brief, light ripples at cruise; crews know these spots well and the movement stays gentle. Above the Middle East 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.