It took me a solid weekend to get everything built. I was able to get a basic LFS system built on a Saturday, and on Sunday I did the "Beyond Linux From Scratch" edition. At one point I got stuck trying to debug a weird interaction between systemd and PAM that took me a while to unravel. That was humbling, I thought I knew just about everything about Linux, but turns out there are large areas where I just don't have a clue.
The docs are well written and maintained, so there wasn't a lot of frustration there. Even if you're not an old hand at Linux you can likely get pretty far by just diligently following the instructions.
I struggled a lot more trying to make a decent desktop environment than I did getting the OS setup. I spent so much time trying to get a nice-looking toolbar (polybar) and basic stuff (like how patched fonts work) took me an embarrasingly long time to sort out. I also didn't know what a compositor was, or why you might want one. I enjoyed figuring out the basics of compton, which allowed me to get cool transparent backgrounds on windows[0], although I never did quite figure out how to get rounded corners.