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 What is your return policy? How does the sound of the Uno compare to the Dohm Classic? Does the Purest Sleep Diffuser have timers? How can I join the affiliate program?