Well over 1200 hours in Unturned. I’ve been playing it on and off for several years now. For what is essentially a one man show, it’s one of the best free games on Steam that has worked natively in Linux since I left Windows over 5 years ago. There are lots of ways to play and tons of mods out there as well. Like most zombie survival games today, once you get the feel for the survival part you can then expand on crafting and exploring. Plus the simplistic look can be a bit more kid friendly with things like rag dolls and blood turned off. My nephew loves spawning in jets and blowing things up.
The guy has also been working on a new version using Unreal engine instead of Unity. No idea if it too will be free, but his blog has a bunch of videos on the progress if anyone is interested -
https://blog.smartlydressedgames.com/unturned-ii-devlog-026/