It can take anywhere from 12 to 24 hours to get into a state of partial ketosis. During a fast, once your serum glucose level drops by 20% your liver will start producing ketones to supplement the energy your brain needs.
Achieving full ketosis can up to 10 days on a ketogenic diet. Upon entering ketosis, some people notice a fruity smell on their breath, a decrease in appetite or lethargy, while other people don’t experience these symptoms at all. Remaining in full ketosis over time requires consuming fewer than 30 grams of carbohydrates per day.
The metabolic benefits of intermittent fasting, however, don’t depend on achieving a state of full ketosis while fasting. In studies of healthy obese individuals, Dr. Krista Varady has observed insulin resistance levels decrease by up to 40% when these individuals stick to an IF regimen for just a few weeks.