Thanks for your question!
I think the demo is about 30-60 minutes, depending on how quickly you read and how quickly you can plan your day. Most people die the first time they play through, so if you were trying to get to day 5 without dying, you might need to reload and try again a few times, which adds a bit of extra play time. There are also some scenes in it that will only happen if you choose for the protagonist to do an activity at the same time as Kim or George, so if you were trying to get all the scenes, so getting all of that content also requires trial and error.
The final game will probably be about 3-8 hours of gameplay, again depending on play style and whether you want to get all endings, etc. It's really hard to estimate, actually, as I haven't timed people who play it for the first time (yet). It's based on roughly 60k words of VN scenes, plus the (challenging) survival aspect. If you're only interested in the story, and are a fast reader/skin reader, it might only take 2 hours to play through on VN-only mode.
TL;DR: 30-60 minutes for the demo, 3-8 hours for the full game with survival, or 2-4 hours for the full game in VN-only mode.