What you are talking about is fasting. There are different kinds, and to reach a true Ketosis state you really have to go at least 72 hours without food, which is the limit you would do outside the care of a doctor, as some can using fasting for up to a month to cure virtually every ailment except terminal cancer. It's best way to eliminate toxins and your body, and let the body fix itself. Most people in the last 100 years think fasting is unhealthy, but taking all kinds of prescriptions and medications is somehow healthy.

What you might be more interested in intermittent fasting which turns on the survival mode in your body. This is not eating for at least 16 hours, generally up to 24 hours and at least once a week, no more than three times, and not on a standard schedule because your body will learn the system if you do it the same way, same day, every week. Most generally fast like this two days a week. This will bulk up your muscles if you are working out. There are different ways to do this, Lean Gains program is the most popular if you look it up. If you getting good sleep with the Primal lifestyle, your skin will start to look better, and if you are a guy your testosterone level will rise some.

Before farming, and the premise of the paleo/primal lifestyle is that people hunter-gathers, not always successfully, having to go hungry many days. There are just too many pro's with fasting, the forgotten best way to help your not only look good, but in experiments with mice, the fasting ones lived about 20-30% longer.

Before you start any kind of a fasting program, do your research on it. Plan, and do not over do it. Some people get so excited when they see what's happening, they get addicted and take it the next level before it turns into an eating disorder. It can be that exciting.

Just don't fool yourself if you are going to do. Don't juice or cheat. Stick with drinking a lot of water. After the first day your hunger feelings will go away if you drink water every time you get hungry. If you think you cannot do it, just practice, and once you make a day, you will see it's actually not bad at all. The problem is that we are so programmed into thinking we always have to eat, that muscles go first like the do with extreme dieting, because they restrict calories, not actually go into a fasted state. Many who go on the medical fast of plus four days typically lose their hunger pains all together.

Make sure that you have a backup plan for food, like Watermelon or some healthy snacks if you get too dizzy. Don't workout or do heavy lifting during the fast. Some will workout or walk just before they come off the fast. If you ever feel like you are getting too dizzy, make sure you sit or lay down for awhile.

If you complete a long fast, you might notice that food taste better as well. Taste buds will be 100%.

Good Luck

Your question will be best answered in detail by reading Volek's book on low-carb performance. You'll only know at what carb level you personally stay in ketosis by doing blood tests. For most people however that number is <50 g carbs a day, every day. Some young and very active men can stay in it at about 60. The ultramarathon crowd will probably still make it at 80 a day. But it's N=1 here, so test.