I believe it says in DANDR you can continue induction as long as you want.

I found this on the Atkin's website:

"A lot of people think of Induction as lasting only two weeks, but it can be followed for a longer time. If you have a lot of weight to lose or have difficulty losing weight, you might want to do Induction for quite a while. That way you'll see dramatic progress before moving on to the more moderate phases of the program."