Could theoretically stay up even longer...Solar aviation is still in its infancy, but it's getting better all the time. We might never see big passenger airplanes being solar powered, but there's all kinds of other cool stuff that can be done with this technology. A good example is what Titan Aerospace is doing with its high-altitude solar-powered unmanned plane.
As Spectrum writes: "The Solara is intended to loft a payload to 20,000 meters [60,000+ feet] and then keep it [there] for five years, running entirely on solar power. It functions a bit like a satellite, except substantially cheaper and much more versatile. And, you can get it back when you're done."
Applications for these types of solar planes are very varied, from disaster rapid response to anti-piracy (the real kind, out at sea) surveillance, to crop monitoring, fire monitoring, providing internet access to remote regions, all kinds of scientific missions (ocean monitoring, cosmic ray monitoring, atmospheric science, weather monitoring, etc). Basically a lot of things that you can do with a satellite, but cheaper, and if your mission is limited in time, you can then land the plane and change up the equipment instead of having a useless/obsolete satellite in orbit.