To reach a ketogenic state, the term fast from a clinical sense required 24-36 hour period without any caloric intake; strictly water. I have done 3 day water fasts, the hunger waves need to be controlled the first 2 days, but the feeling is gone after that point.
There are many things to consider, including refeeding syndrome, it is highly advised to seek MD advice prior to undertaking such a diet.
Lastly, there is a ton of research highlighting fasting and its effects on stem cell regenerative properties.
One thing that surprised me when water fasting was that while the hunger went away on the second day (starting in ketosis helps), I found myself still craving the reward and ritual of meals. Lunchtime arrived, and I missed the excuse to take a break from life/obligations and indulge in the enjoyable process of eating, independently from satiating hunger. I think when I do it again, I'll find some non-consuming activities to fill that void.
There are many things to consider, including refeeding syndrome, it is highly advised to seek MD advice prior to undertaking such a diet.
Lastly, there is a ton of research highlighting fasting and its effects on stem cell regenerative properties.