How long will my Helium-filled balloons float?

While it's the small size (and hence the"lightness") of the Helium molecule that causes Helium-filled balloons to float, this attribute also creates a pesky problem when trying to keep Helium-filled Latex balloons inflated for long periods of time. Because Helium molecules are so much smaller than the space between the molecules of stretched Latex rubber, Helium will slowly seep out of a Latex balloon directly through the balloon's wall. This is why untreated Helium-filled Latex balloons only stay inflated for a relatively short period of time as compared to air-filled balloons.
On average, untreated 11 inch diameter Helium-filled Latex balloons can be expected to float for about 12-18 hours, and 16 inch diameter Latex balloons will float for up to 24-36 hours after being initially filled (your "mileage" may vary depending on temperature and atmospheric conditions!) Treating the inside of Latex balloons with a water soluble, transparent plastic material called Hi-Float® will greatly help them hold their Helium longer. This product works because its molecular structure (plastic) is much "tighter" than Latex rubber, so it reduces the rate of Helium loss through the Latex balloon's outer wall.