According to About.com, achieving a consistent, ideal level of nutritional ketosis generally takes two to four weeks. Longer time frames are often due to the fact that some diet adjustments may be required to hit target ketone levels.
Medical News Today explains that ketones are carbon fragments that the body produces to use as fuel when it has burned up its supply of glucose. Instead of fueling itself with carbohydrates, the source of glucose, the body begins to draw its energy from fat reserves. Ketones are produced by the liver as it processes fatty acids and show up in the bloodstream as a result of this process.
To achieve nutritional ketosis, a diet low in carbohydrates and high in protein and fat is required. There is some debate about the safety of such diets, since very high levels of ketones in the blood make it more acidic, which can cause damage to the liver and kidneys. However, Medical News Today states that this risk is generally associated with people who suffer from diabetes, as they are more vulnerable to high ketone levels due to a lack of insulin. The article cites a report from Indiana University School of Medicine which states that low-carb diets do not harm the kidneys. Dr. Atkins recommended monitoring ketone levels while dieting to ensure that they stay at safe levels.