You should know when it's good to stop. As long as you're doing it for reasons of health, and you're in touch with your body, you should be able to know well enough what is good for you.
I'm on day 7 of a 30 day cleanse, then I'll be a raw gourmet for a month, and then I plan to do another long fast in June. I have a 15cm ovarian cyst to get rid of, so I know I have to take a long process to totally removing it. The weight loss is just a nice side effect.