There is many theories about fasting. I personally like it and recommend to some customers sometimes. It all depends on personal circumstances. Fruits and veges fasting for a few days is fine but if you want 2-6 weeks plan consult a nutritionist or your doctor before you put your body on a starving mode.
Please do not follow any popular fasting diets you can find online or in magazines. We are all different so what is good for one person may not be good for you.