I have 3 foil party balloons still inflated after 2 months and 3 days. I left them by the window as heat from the sun provides kinetic energy to the helium atoms to improve the balloons longevity. These three balloons have provided me with enough joy to keep me inside staring at them all day not outside at risk of injury which ultimately leads to an unnecessary MRI.
My father turned 80 and they kept his mylar balloon around in the living room for at least 18 months. It's one of the few things that survived the cat.
Gas pressure is the atoms/molecules bouncing off something else. If the atoms have more energy, then they impart more energy into whatever they bounce off (inside of the balloon), which essentially means higher pressure i.e. the balloon appears inflated again. Until the sun goes away and the OP's party dies for a time. :)
