I think that 60 carbs is what is in the Atkins book as the suggested level to go to for ongoing weight loss, however is some of the other plans here that are not classified as low carb but rather, controlled carb the level is higher. Sometimes up to 150-180 a day depending on the person and the plan. With the ongoing weight loss in Atkins a person is supposed to gradually increase their carbs to find out what level they can eat at and continue to lose.

Edit - and those levels are more left up to the individual to realize for themselves rather than a set number, which I think is what you wanted to know. A lot of people are starting to find out they can go into ketosis at over 100 carbs a day even though their plan does not require them to be in ketosis.