Prune juice takes different amounts of time to relieve constipation depending on the severity of the problem. Prune juice starts working within an hour and can take an hour, six hours, a week or several weeks for a lazy bowel, according to Total Health.
Prunes are rich in sorbitol and fiber, effective stool-loosening stimulants that diminish the transit time of stool, thereby relieving constipation. A serving of juice should pass through the intestines before additional amounts to avoid overboard consumption. A Sudden increment of prune juice can increase the time required to move the bowel and other unpleasant side effects such as diarrhea, cramping, and intestinal pain, states the University of Mississippi Medical Center.