Istanbul to Miami is about 9,583 km and usually takes around 13h in the air. The track it follows runs over the Balkans, the Alps, Central Europe, Western Europe, the British Isles, and the North Atlantic.
Air lifting over the Alps can raise brief, light ripples at cruise; crews know these spots well and the movement stays gentle. Over the Balkans, Central Europe, and Western Europe the air is usually calm and settled. Nearer the British Isles and the North Atlantic 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.
A flight this long spends hours at a high, smooth cruise. Even when one stretch is bumpy it is a small slice of the whole trip, and the aircraft treats it as routine.