One thing I will say about ketosis and a low carb diet is that so many people give up when they are going into ketosis. Not sure if you all know this but thought I would post!!
When you go into ketosis you can get shaky, headachey, feel generally weird with a fuzzy metalic taste in your mouth. A lot of people mistake this for needing carbs and they can't cope anymore on a carb free diet but if you give it a couple of hours the feeling will pass, you will be in ketosis and then you are on a roll. I have seen so many threads/heard from so many people who said they couldn't possibly continue with a low carb diet because it didn't suit them, made them weak and shaky with headaches etc which I am pretty sure was just the start of them going into ketosis - if they had continued for a few hours more they would have made it into ketosis and started feeling great!
Getting into ketosis depends on how much glycogen you have in your body but usually for someone who has been eating carbs regularly everyday it takes 3-4 days, glycogen is an energy store contained in the liver. When you start a carb free diet your body uses up the available carbs first, then it uses the glycogen which can weigh 5-7lbs and then when it has used up all the glycogen the body starts working on the stored fat in your fat cells.
Many people have an amazing first week when they start a carb free diet as they lose the 5-7lb of glycogen, however you have to bear in mind that the minute you eat carbs again you will regain the glycogen as it is not really fat lost. Therefore it is best if you aim to go 5lbs over your target weight in order to compensate for the glycogen that will be replaced later.
For myself and my husband we have had carbs over the last 4 months, we might have a carby weekend and then back to no carbs on a Monday, because we are generally no carbs if we do eat some and knock ourselves out of ketosis we can get back in within half a day or so.
The pig to twig guide is 20g of carbs a day in phase one, however I know I can eat a lot more than that i.e. 60-80g of carbs and still stay in ketosis. However another thing I have noticed is that I can stay in ketosis but if I eat too many calories I still will not lose weight so being in ketosis alone is a bit of a myth hence why a lot of people failed at Atkins, they were eating too much protein, calories fat etc. Personally for me to lose weight effectively I keep an eye on calories as well as carbs. Pig to twig do say not to worry to much about calories so I don't think you should be counting religiously but just thought I would share my experience.
One other thing, being in ketosis means your body is producing ketones which need to be flushed out so drink, drink, drink the water.