For what it's worth, in the real world, skywriting is usually done at around 10,000 feet and is visible in an up-to-15-mile radius. The spell could potentially be at a higher altitude, but then you have to use a larger font size to keep it readable from the ground, so I'm not sure how much you gain from it.As far as I know, Skywrite is effectively the most far-reaching of AoE spells.
The spell simply says that "You cause up to ten words to form in a part of the sky you can see", and that, "The words appear to be made of cloud".
No direct reference is made to height but, with reference to sky and a resemblance to cloud, we might apply potential heights of several thousand feet and up to well over 40,000 feet (as related to Earth-based atmospheric physics)."
D&D Beyond says only 1 hour (which is still better than real-world skywriting that lasts maybe 20 minutes).The spell's description states: Duration: Concentration, up to 1 day, ...