It only takes 1-3 days to get into ketosis. But getting in ketosis is easy, becoming keto-adapted is the tricky part. It usually takes a month to get to the first stage of becoming keto-adapted, and it takes up to 2 years to fully train your body to use ketones fully.
I am a female with a lean body mass of about 88 lbs (normal total weight around 120-130) and it takes me 24-36 hours to get into ketosis. I can tell with ketostix and by this fuzzy almost headache I get for the first 12 hours or so of it in the front of my head. This is if I cut down to 30 net grams of carbs or fewer (which excludes fiber carbs.. Atkins calculation style).
Two-Four weeks. Even if you take a day of break from ketosis, you will have to go through another two-four week re-adaption.
The ketogenic diet is a lifestyle change for people who are serious about eating to live rather than the other way around.
Here is a differently styled answer. Your body will enter ketosis once it has exhausted all of your blood sugar and all of your glycogen stores. It will also not happen until you body is able to keep your blood sugar and insulin at normal levels, if your diabetic, this can take some time.
The body is always producing at least some small level of ketones. But the ketones themselves really aren't important, it's what their levels signify (fat breakdown). Once the available blood glucose and stored glycogen is used up, the body starts breaking down triacylglycerols (the fat we all hate) to use the resulting fatty acids for energy to get gluconeogenesis going in order to stabilize blood glucose levels. The results from this oxidation of triacylglycerols are ketone bodies. all of these metabolic processes are constantly occurring all at once to some degree, just some way more than others depending on the levels of insulin and other hormones. I just am looking for the specifics like what concentration of glucose is enough to signal the fed-state (when the body stores fat,) and at what rate the body uses the blood glucose it has. The reason a key symptom of untreated diabetes is rapid weight loss is due to them not making the needed insulin. Insulin is one of the key regulators for the fed state, so therefor their body chemically always thinks it is starving, even if there is abundant blood glucose, so it's always breaking down fats for energy to synthesize new glucose from precursors and tearing down muscles for amino acids. Once they take enough insulin, the body can then enter the fed state and start storing fat and rebuilding. So what I want to know is what amount of glucose at any given time is enough to raise insulin to the level needed to signal the body to store fat. I also realize that with a calorie deficit you will also lose some of this stored fat over time though. It's funny that none of my dozens of biochem books ranging from undergrad level up to phd level give these quantitative details.