The length of time that a helium balloon will float depends on many factors that include inflation level, temperature, size and material of the balloon and if it is sealed with a product like Hi-Float. For example, a latex balloon that is well-inflated and is 11 inches will float between 12 to 24 hours. However, if this same balloon is sealed with Hi-Float can last between 1 and 4 weeks.
If it is a mylar balloon that is inflated, it can float for up to 5 days before deflating a little. However, it can still float for another 2 weeks.
Another factor like temperature can affect how long balloons float because helium is a gas that contracts and expands according to changes in temperature.