There are some examples of humans being in ketosis for extended periods without ill effects.

As Rose mentioned, the Inuit probably lived more or less their whole lives in ketosis, or at least most of them were in ketosis most of the time. They weren't the only such culture, either—maybe the Masai and maybe the Bison people of the American Great Plains did likewise, at least sometimes.

There there was the one-year-long experiment in which Vilhjalmur Stefannson and another subject ate nothing but meat while under medical observation.

Then there are the thousands of children who have gone on a ketogenic diet to treat their epilepsy. Many of them stayed on the diet continuously for years. In Caraballo 2011 they reported on more than 200 patients who had been on the diet for between 1 and 12 years, with a mean of 3.5 years. In that and in other studies, it seems like there weren't any common, bad side-effects. (There were some side-effects, but they weren't too bad or too common. The worst was stunted growth, but I think that was not due to ketosis but due to the fact that those diets were usually calorie-restricted and water-restricted, and children need calories and water to grow!)

Also, you have been in ketosis for a year or two straight. Before you started eating solid food.

Also there are the modern group of zero-carb eaters, of which apparently Rose and my wife Ambimorph are the representatives on Paleohacks. They hang out on a forum named "Dirty Carnivore" and some of them (used to?) hang out on a forum called "Zeroing In On Health". There are apparently dozens of them, at least, who've stayed continuously in ketosis for at least a year.

In short, I've looked for evidence that being in prolonged, continuous ketosis is harmful and I haven't found any. I don't see any reason to worry about staying in ketosis.

I've been in ketosis continually since April of 2007, except for a brief stint on Kwasniewski's Optimal Diet, which at 60g of carbs or so kept me above my threshold (as Aaron notes in his answer, levels vary between people). My health has done nothing but improve since then, including my lab work (pardon me while I take a moment to gloat over my doctor's astonishment--hee!)

I've heard warnings about ketosis being "hard" on the body, but nobody's ever produced any evidence of such difficulty. Ambimorph asked the question directly here on PaleoHackers, "Why do people consider ketosis stressful to the body?" The bottom line seems to be that people just imagine it must be, because it's not non-ketosis, lol.

And I know the Inuit have been discussed endlessly, but regardless of how much plant matter they ate, there's no doubt most lived their entire lives in ketosis prior to the introduction of European foods.

From what I understand reaching ketosis is eating below 50grams of carbs a day (could be lower or higher we are all different). This means we are eating pretty much all meat. I believe it was Sisson who said their are some LC veggies and fruit (vitamins and minerals) that the body needs. When I have reached ketosis in the past I never stayed for more than a week at a time because I felt i needed some more carbs. I'm anxious to hear more answers also. I don't think we have to be in ketosis to burn fat all the time but would like to hear what others think.

From personal experience, I've been in ketosis for about a month straight. Especially, when I'm traveling it's a lot easier to just eat meat and fat and not worry about the veggies too much. I worry about ordering starches at a restaurant because they cook them in cheap oils. I usually bring some coconut flakes and snack on that often.

I don't think there is an exact limit on how long you should stay in ketosis but I know that after a while I tend to develop a slight episode of depression and tiredness. I just get lethargic and know it's time to eat some carbs to jump out of ketosis temporarily.

However, after being on it for a bit, most of the typical ketosis signs disappear. No funny smelling pee and no metallic taste in the mouth. It just feels normal, but you definitely feel like you're running on fat fuel.

What about, however, after you have switched over to burning ketones for fuel and are no longer "shedding" them. Would you technically be "in" ketosis then? i.e Nora Gedgaudas and others discussion after switching over from sugar burning for fuel. I don't know the answer but I know that my breath and so forth after the adjustment phase are normal/better and my insulin levels stay more constant. Science geeks please weigh in! What would the keto-stix say after adjustment from sugar burning to fat burning? (Zooko..interested in you or your wife's answer. )