That is a common response. Many people don't get the "butt pea" until 10:00 am the following day, sometimes even later at 3:00 PM. Online information gives reaction times from 30 minutes to 12 hours for Epsom Salts . This is dependant upon a number of factors. In experiment only 3 out of 8 people had watery diarrhoea with 3 hours after taking Epsom Salts following a 10 hour fast.