Of course it depends on your system but it's more a matter of minutes than of hours. It's a really enjoyable experience to hack on the system and being able to rebuild in seconds. Just have a look at some SerenityOS hacking videos, they are truly inspiring.

From my experience building it under WSL on a reasonably recent laptop: from cloning the Git repo to seeing SerenityOS pop up in QEMU, maybe half an hour. A lot of it is downloading and building the toolchain.

And then incremental rebuilds (when I go and pull the latest changes once in a while to see what's new) take anywhere between 1-10 minutes, depending on what changed exactly. YMMV.

For an entire operating system, it's pretty cool already! I wonder if C++20 modules will take it down some more, it's been mentioned a few times as something to investigate.