My longest Master-Cleanse was for 21 days, but a friend of mine was on it for 31 days.
She had a severe lungs "unknown" infection that could not be killed with the strongest Antibiotics . The doctors gave up on her, as the infection had spread on both lungs.
My friend was constantly coughing, had problem breathing, had a constant high fever. She thought she was dying. She was only 30 years old, mother of a 3-year old son.
I convinced her to go on a MC.
She did it for 31 days + 7 days juice fast.
When she went back to the doctor and he compared her catscans from before and after MC, he went speechless. The infection was gone.