On average, it takes approximately 16 hours to prepare for a colonoscopy procedure. The prep for the procedure takes longer than the actual procedure, according to Harvard Health Publications.
To undergo a successful colonoscopy, a patient must completely empty the contents of his colon, states Harvard Health Publications. To accomplish this, he may have to follow a special diet or drink a bowel prep liquid. Most often, the patient drinks a bowel prep liquid that triggers diarrhea to clear the colon of its contents. A patient typically drinks up to 1 gallon of bowel prep liquid to ensure the colon is completely clear.