How much curvature you can see depends on how much of the horizon you can see. Warning, crude calculations ahead!
If you can see all 24,901 miles of Earth's horizon, you will see 360° of curvature. If you can see 6,225 miles of the horizon, you will see about 90° of curvature. At the cruising altitude of a typical airliner, the horizon will be about 230 miles away. That gives 460 miles if you have a 180° or greater field of view. 460 / 24901 * 360 = 6.65°. So at cruising altitude, you will see a maximum of about 6°.
But looking out a passenger's window on an airliner, you will see only about 45° (with your nose pressed up to the glass), not 180°. This will limit the viewable curvature to about 1.66°. That is why I doubt people when they claim to see curvature from an airliner. The pilots, maybe. If they have a "good eye" for subtle changes in angles. Passengers, not likely.