Normally I'd say less than 2 to build it, and install the OS and drivers, and get the OS fully updated if I've got broadband. Less than 4 if we talk about tweaking out the OS to get all my personal preferences set up, and install all the software I need, do a defrag run after it's all set. (Add another hour for time to get a list of all the software I needed before the install started, download all the shareware/freeware, make backups of my mail files and settings for all the software I could. Forty-five-plus freaking applications and utilities I had to download to match the ones I had before for this new build, plus the other software I have the install CDs for.)
The build of the machine I'm typing on now took about 12 hours, including the initial list compilation and software download time. Also including the time for ghosting the hard drive to a backup disk since I was re-using the drive and for a recovery in case something went horribly wrong, as well as wiping the drive before re-use, as well as a few Ghost backups of the OS during setup and installation so I could fall back if I screwed something up, which I did and which the backups didn't help at all. The actual hardware build only took an hour, including removing the existing hardware from the case I was re-using, and cleaning the case out.
I ran into some problems with software and hardware compatibility with this build though. Discovered that Ghost would NOT run when I had the hard drive plugged into the SATA ports. I assume that this was due to the IDE to SATA adapter I'm using since I didn't want to buy a new drive. If I plugged it into the IDE port it worked fine (only one IDE port on the board and I don't want to have the HDD and optical drive on the same cable); so every time I made a Ghost backup I had to disconnect all the drives and put it all back. In hindsight, I should have just left it on the IDE cable during the installation.
I also had to reinstall Windows 3 times, due to issues from using XP MCE and trying to make it work without the MCE software.