The Forever Living C9 diet (previously known as the Clean 9) is meant to be followed for only 9 days. After that, you're supposed to increase your calorie intake and follow the F15 diet for 15 days. Finally, you'll continue with a maintenance program known as V5 (Sports Performance).
Considering how restrictive it is, you should not follow the Forever Living C9 diet for more than 9 days. If you wish to try it again, it's best to wait a few weeks to make sure you've provided your body with enough nutrients and calories.