I run gentoo but without the UI...
It took me 2 days to get it running on my Pentium 166 (with MMX yeah!
)
But I slept in between, it probably took 24 hours total.
It depends on your download speeds, and processing power.
A good choice on the installation docs btw. They are very adequate for installing. I consider myself a moderately experienced linux user and they got me through it in one time.
Have fun!