Istanbul to Los Angeles is about 11,002 km and usually takes around 15h in the air. The track it follows runs over the Balkans, the Eastern European plains, Central Europe, and Scandinavia.
Over the Balkans, the Eastern European plains, and Central Europe 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.
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.