How long will the Purest Sleep Diffuser run before it runs out of water? January 23, 2019 19:37 The Purest will last between 3-6 hours depending on the selected intensity level (low or high). Related articles How do I clean my Purest? Does the Purest Sleep Diffuser have timers? Are the Sleep Scents safe for use with toddlers and children? What is your return policy?