I did a 3 day fast earlier last year to reset my immune system. It takes about 72hrs for cells to reset. That one was pretty easy. This 7 day was tough. I had headaches adn didn't feel right until day 6. I'd been drinking a lot, so I think I was dealing with some withdrawal. You're only hungry for the first day or 2. It comes in waves and is gone after 10min or so. Day 4 and 5 were tough, because I wanted to eat. I wasn't hungry, I just missed it, and you can really smell food when you fast.
I'm back up to eating between 6-8hrs each day (Intermittent Fasting) Usually 12-6p, weekends I relax a bit and have some whiskey (ZERO CARBS!). Had my first beer during the games since 1/1. It was delicious. I'm trying to keep my carbs down and burn fat, but one cheat day puts you out of keto and it takes about 3-4 days to get back in without fasting. Not necessarily trying to stay in keto always, just enjoying some benefits from it.
I'm back up to 170, and here is my spreadsheet I kept on my fast, if anyone is interested in this kind of stuff:
https://drive.google.com/open?id=1iDSbWuRYPjSmvIxEUNmw4c-Pd49tEIKCZWCguk7DhiY
And yes it was a water fast. So I started the first 3 days on only water. Day 3 I started adding electrolyte powder to my water (you can get it from GNC, and it helps inhibit muscle atrophy). Day 4 I started drinking flavored seltzers and adding lemon, lime, and cucumber to my water for flavor. Didn't eat any solids. I also had an herbal tea in the evening (when I normally would have a beer/whiskey). I made sure my teas didn't have caffeine, I did't want any other effect other than flavor.