Latex balloons can last up to 8-12 hours without hi-float and 12-24 hours or more with treatment with hi-float liquid. On the other hand Mylar balloons can last for 5-7 days or even months depending upon the quality. To sum up the whole discussion, if you have a long event stretching beyond one-day duration, then go with foil balloons. Even if you use latex balloons, then go for high quality, treated with hi-float liquid and large sized balloons for the event. Inflate the balloons right before the event to avoid any partial drooping or deflation.