From Atkin's site:
How long can I stay on Induction?

The longer you consume no more than 20 grams of carbs daily, the more body fat you will burn. Depending on how much weight you need to lose, you can safely continue with Induction as long as the following three conditions are met:

Your blood chemistries, lipid values, blood pressure or blood sugar levels continue to improve or remain stable and within normal limits.

You feel well and are experiencing a high energy level, normal sleep patterns and stable moods.

You are not bored. Boredom could lead to cheating and undermine your efforts.

However, it is important to understand the entire Atkins Nutritional Approach. The ultimate goal of the program is to advance from the Induction phase through Ongoing Weight Loss and Pre-Maintenance, culminating in Lifetime Maintenance, which should become your permanent way eating. By following these steps, you can find your Critical Carbohydrate Level for Losing (CCLL), also known as your carbohydrate threshold for losing, and ultimately your Critical Carbohydrate Level for Maintenance (CCLM), also known as your carbohydrate threshold for maintaining. Segueing from one phase to another will help you maintain a healthful weight, feel good and decrease your risk factors for chronic diseases such as heart disease, hypertension and diabetes.

That being said, if you have a great deal of weight to lose, you can certainly stay on Induction for six months or even more. When you switch to Ongoing Weight Loss, your rate of loss will naturally diminish. On the other hand, if you have a modest weight loss goal, say 20 pounds, and lose the first pounds rapidly, it is important to move through the more liberal phases so you can establish the good eating habits that will become part of your ongoing lifestyle and end yo-yo dieting.

http://atkinscenter.com/helpatkins/...nInduction.html