It's always best to install a new operating system on a clean disk, and the news that Windows 7 in-place upgrades can take extremely long is just one more reason why not to install Windows 7 over your old operating system.
The Tech Report spotted that while a clean install of Windows 7 takes as little as 30 to 50 minutes, a Windows 7 upgrade can take up two to six hours, and in extreme cases even over 20 hours:
For a "heavy user" with 125GB of user data, 40 installed applications, 15 "optional components," and modified settings, the upgrade can take 150 to 355 minutes, depending on the hardware—up to six hours, in other words. Meanwhile, a "super user" with 650GB of profile data can look forward to an upgrade time as high as 1220 minutes (over 20 hours) on a "mid-range" system with Vista x86. Ouch.