It'd probably take about a week for a noob to get a reasonable desktop up and running with LFS or Gentoo, but that week will be some of the most valuable education about software build processes and operating system design one can find.
I beg to differ. I don't recall how long it took me exactly, but the only thing I learnt about build processes was, both on LFS and Gentoo: if it fails to compile, lookup on google what to do and apply that. If all else fails, edit the code and hope it does not cause a waterfall effct.And I even cared about learning. If you just follow the manual you'll have learned nothing at all.
I spent nearly an entire summer doing this, back after I graduated from High School in 2003. If you just follow the manual and enter the commands blindly, you can do it a lot faster. Instead, I took the time to look up each package I was installing, figure out what it did, why I was installing it, and so forth. Chipped away at it a few hours a week, then went through the process of install X windows, _which was the worst experience of my life_.
When I was just starting with Linux I did it in a week. Most time was spend on waiting for compilations. Second time hug was exploring and learning about stuff that was mentioned.
If you just want to follow the instructions, it's a no brainier in terms that you can do it from start to finish with minimal prior knowledge.
The instructions are very clear and easy to follow, depending on your specs it takes a few days to compile everything. Certainly a week would be enough time.