Well, I stare at a computer screen for the whole day because it's part of my Job and I feel nothing. I wish I could say the same in case of VR or while using mobile in a moving car.
Havent used VR set myself yet but i can imagine at best you need to get used to the feeling in order not to get sick but ive seen people use it for a long time and didnt complain so i guess it also depend on a person.
Yes, it sure does vary from person to person but, motion sickness is quite common in human beings and there are a few advances to tackle this problem yet nothing groundbreaking.