Depends on if your computer is a desktop, laptop, or server. Depends on the interface speed ATA33/66/100/133 of your hard drive, and the CD drive you are installing from. If you are using a desktop with Pentium 3 or 4 with 128-512MB RAM, using an IDE hard drive 5400-7200 RPM at ATA133 and a CD-ROM with ATA133 @52x.... It should take you about 20-30 minutes.
If you have a CPU less than 500MHz, CD Drive less than 32x, Hard drive 5400 RPM with ATA33/66 and less than 128MB of RAM... I'd say about 1 hour to install.
Also how many plug and play drivers the setup has to search for will add time. So if you are installing on a desktop, unplug all additional hardware during installation so the setup will be quicker.
On a laptop... I have had results from 20 minutes to 2 hours... so it just depends on the hardware.