The time it takes for colonoscopy prep to work varies by individual and the type of prep used, but typically bowel movements start within 30 minutes to 4 hours after starting the prep. Most people experience multiple urgent bowel movements as the colon cleanses. For PEG-based preps, it may take 1 to 3 hours to start working, while sodium phosphate-based preps may take 3 to 6 hours. Bowel prep is usually done the evening before or split between the evening before and the morning of the colonoscopy, with the goal to have clear, yellowish liquid stools indicating a clean colon. The entire bowel cleansing process can take 12 to 16 hours, and completing the prep fully is crucial for a successful procedure.