Toronto to Istanbul is about 8,171 km and usually takes around 11h in the air. The track it follows runs over the North Atlantic, the British Isles, the North Sea, Central Europe, and the Balkans.
Over the North Sea, Central Europe, and the Balkans the air is usually calm and settled. Nearer the North Atlantic and the British Isles 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.