That actually depends. In my case (n = 1) if I stop eating while NOT being keto-adapted, after some period of time I will feel physically weak and mentally slow for a while. That's my blood sugar crashing, but subjectively it just feels as being weak and slow, not "extreme discomfort" at all. I don't normally get serious pangs of hunger.

I don't think I'm typical in this respect, but I would be surprised to learn that I'm only one in a thousand or so.