From The Dark Side--Reinstalling Windows

according to this article:

"To do this task, you'll need a full day [emphasis added], your original operating system installation CD-ROM, a floppy disk and some sort of backup program."

Of course, it only takes 10-15 minutes to newly install the Mac OS. However, a clean install over an existing setup can take a half hour or so. [Dana Baggett via MacNN]