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.
Also... if you are not doing a quick format on your 80gb drive... you might have to add an hour to format it... be sure to use quick format tool is they are available. Most Western Digital and Seagate retail drives come with a CD for formatting and drive utilities if it is IDE. If you bought an OEM drive, then you are at the mercy of the windows 98 FDisk to how long the formatting takes... but usually 80g is around 1 hour.
So on fast hardware using Fdisk to format the 80g... could be 1-2 hours to install.
If you have have formatting utility to do it quickly, then 20-30 minutes to install.
If you are using ancient hardware and the old fdisk to format... you better make a whole pot of coffee... because you will be waiting forever.