[PhasmaFelis]

~6 months is the current best, actually. [1] This is a development project; there's no reason to assume that the current record is a hard limit.

[1]https://en.m.wikipedia.org/wiki/Loon_(company)#Timeline

[londons_explore]

Loon balloons are helium, and helium gas is famously hard to contain, so eventually leaks out and the balloon comes down.

I don't see any way to make a balloon stay up forever unless you have a particle accelerator to manufacture new helium to make up for leakage...

[ProblemFactory]

> Loon balloons are helium, and helium gas is famously hard to contain

It's not like there are any better options? The only two practical lifting gases are hydrogen and helium. Hydrogen leaks even more than helium and is flammable, even though it is much cheaper and abundant.