On hot days, in direct sunlight, balloons may pop within an hour. The same result can occur indoors if balloons are exposed to direct sunlight through windows so please be keep this in mind if transporting balloons in your car.
When it's cold out, helium molecules shrink, reducing the size and float time of balloons. Don't be alarmed if you're balloons shrink a little bit when going from a warm place to a cold place. Once temperatures warm up, generally the balloon will expand.
Indoors or outdoors, the constant flow of air around a balloon will increase the speed at which helium molecules escape through the wall of the balloon, resulting in reduced float time.