The pathogenesis of ectopic gastric mucosa is a matter of debate between a congenital-embryological-and an acquired origin-consecutive to a gastroesophageal reflux (GER) like Barrett's esophagus -, the most commonly accepted theory being that these areas are embryological remnants.
Standard size latex filled helium balloons stay afloat for approx 8 - 12 hours, metallic helium filled balloons float for 2-5 days. I have found that a 36 latex helium filled balloon can stay afloat for a number of days due to the thickness of the balloon and the massive quantity of helium.
Flying times will depend on the size and quality of the balloons, and the conditions on the day you inflate them. A standard size helium balloon is 28cm (11) and should fly for 12-18 hours. Balloons are adversely affected by heat, wind and sun, so keep them in a cool, dark place until needed.