I've done three days a couple of times. I eat a big meal the night before I start at dinner time. For some reason, that makes Day 1 go easier for me. I've tried easing into it, but it makes me really hungry Day 1. I usually get a huge headache at the start of Day 2 or so (probably because of low sugar and no caffeine), and get really hungry. By the end of Day 2, you're making all sorts of excuses on why this was a bad idea and that you don't really need to do it. Day 3 is weird because, for me, I find that people start to offer me food. Like my friends will say, "Got an extra apple pie. Want it?" or "My treat." That hardly ever happens when not fasting, but almost always when I do.
Oh, your urine might get much darker on Day 3. That's your body taking advantage of down time to clean house a little. When a person is eating, your body has to put a lot of energy into the digestive system (think how big that thing is inside you!), but during an extended fast, your body can redirect some energy to spring cleaning.
Usually the stomach rumblings stop pretty early on, but the cravings are pretty intense. I get light-headed sometimes during a fast. And I see food everywhere. You become intensely aware of just how much advertising of food is being done.
Now, coming off a fast has to be done right. Three days isn't that big of a deal, but if you think you're going to just go slam a full meal, you're going to spend the next several hours feeling absolutely barfy. You have to work into it. Have part of an orange. Wait an hour. Have a cracker. Wait an hour. Have a clear drink and maybe part of a bread slice. Wait a couple hours. Have some tomato soup. You have to first get your digestive juices turned back on, which means that you have to send some food down the pipe, but you don't want to cause a shock to your system! By the end of Day 4, you'll be back to eating just about whatever you want, but I'd still avoid huge greasy meals until Day 5.
If you fast longer, you have to allow more restart time.
