Neowin posted a news story that it can take up to 21 hours to upgrade to Windows 7

With the official release of Windows 7 drawing closer every day, many people are preparing their systems for the change. A vast amount of more tech-oriented folk are planning a clean install of the upcoming operating system from Microsoft, whereas the rest are obviously heading for the upgrade route… one which could potentially cost them a lot of time. After a fair bit of testing the upgrade system, some rather interesting conclusions were made, which can be found here on the blog of Chris Hernandez (a Microsoft Software Engineer). Apparently, if you're a "Super User" (meaning you have over 650GB of data and roughly 40 applications), it could potentially take you almost 21 hours to upgrade to Windows 7.

Windows 7 can take up to 21 hours to upgrade