I've been using Linux for about a year, and as my main Desktop OS for about 5-6 months now, I've downloaded, checked and burnt the Linux From Scratch Live CD, and in about a week I'll be off school for the summer.

However I have a few questions.

Firstly, and not really related to LFS, should I bother with an external hard-drive for LFS?
I have enough space on my main hard-drive, just about, but 3 OSes on 80GB might be pushing it a bit.
And I think it'd be easier and safer to make LFS on a fresh drive instead of trying to tri-boot safely and stuff.
The drives I have in mind are USB 2.0, either 160GB or 250GB, so space doesn't matter, just speed.
Would it slow me down a lot, since LFS involves a lot of compiling?
And would it be possible to boot from successfully and fully, as if I recall correctly everything in Beyond Linux From Scratch is done while booted from your LFS.
Also it might be nice to actually use it once or twice once finished, if only to show off.

Secondly, how much time would it take to set it up?
I understand that getting a basic console-based Linux up would be a lot quicker than setting up a windowing manager and stuff, so I'm really asking two questions...
How much time to just set up LFS?
And how much time to get a window manager up and usable (if I get that far)?