You are mixing two different things.
1. Motion sickness When you move in the VR, the brain visually perceives a movement, but not the inner ear, which is responsible for registering bodily movements.
2. Flashing pictures: Recent research indicates that gamma waves (> 30Hz) play a significant role in the brain.
The flickering frequency on the screen stimulates the visual cortex, which can cause discomfort, headaches or epileptic seizures.
But the first symptom should appear almost immediately. And that there was a second symptom, you probably twelve hours continuously look in VR. Therefore, in glasses and don't get seasick, but only strain the eyes. The same can be from the normal screen.
That's my guess, I'm not a doctor. In General, I think that the harm of VR is exaggerated. It only hurts those who abuse it.
Actually, i think people who used the earlier versions experienced these issues mostly and even everything better right now, still they are talking about their bad experience back then.