As far as I know, it takes 21 days of fasting to heal ulcer. Three years ago I did 23 days of Water Fast and during the fast I felt wonderful, but then, breaking the fast, I made a mistake and ate food I wasn't supposed to eat so soon, and then I had problems. The fast is OK, the longer - the better, just make sure you break the fast well. Usually, breaking of the fast lasts as long as the fast itself.