As 'soontobegorgeous' says, it really depends on the person. I did Atkins for around five months prior to cd, so i'm quite hot on the whole carb thing! Everyone has different levels of tolerance - some people find that simply having the bars (which are higher in carbs than the shakes/soups) kicks them out of ketosis, and I even have one client who's never got into ketosis on cd, and is constantly hungry. Other people can have as many as 100g of carbs per day and remain in ketosis. It's certainly worthwhile having some idea of what your carb tolerances are, but it's perhaps an experiment that should be saved until after you've got to goal!
So, the 'short' answer is that your carb limit for ketosis is probably somewhere between 25g and 100g per day!