Allegedly, the day on Jemison is 49 hours long. IIRC, one visit there, I experienced a full day/night cycle, despite not having played even 49 hours yet, so I'm guessing the former.
On a more positive note, preliminary tests on whether the skybox is "real" are looking good. Here's snapshots of a local day on a moon around a gas giant, taken three local hours apart... which can vary between 39 minutes to six hours, depending upon the body you're on. This moon is tidally locked, quite close to its planet, and is apparently in a slightly inclined orbit.
In contrast, here's some snapshot planet around which the habitable moon of Kreet orbits. There's a second moon, closer in. I generally had to wait 28 local hours before the planet moved into view, and there were several local days where it didn't come into view at all, so when I saw it barely pass into view near sunrise, I ended up having to get up off the bench I'd placed, and look at it directly. You can see the inner moon getting larger as time passes in the first three snapshots.