Helsinki to Istanbul is about 2,180 km and usually takes around 3h 30m in the air. The track it follows runs over Scandinavia, the Eastern European plains, the Black Sea, the Balkans, 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. Over the Black Sea, the Eastern European plains, and the Balkans the air is usually calm and settled. Nearer Scandinavia the weather turns more Atlantic and the ride can be livelier for short spells, still firmly within the routine. 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.