The upgrade to Windows 7 could take all day and night, literally.

While we're not going to dispute that a clean install is the way to run Windows 7, there are going to be many users who will choose the literal upgrade path from Windows Vista and perform an in-place upgrade.

The reasons to do an in-place upgrade are numerous. First of all, the core software changes to go from Vista to 7 aren't as drastic as previous generations of Windows. Secondly, the convenience afforded by the in-place upgrade allows users to retain nearly all software and settings without the need to restore any previously backed up data.

Another possible advantage to doing an in-place upgrade is time – it's supposed to be faster than starting fresh and reloading all your old programs. But in some cases, the upgrade could be the more time-consuming route.

Microsoft's Chris Hernandez of the Windows Deployment team detailed Windows 7 upgrade performance as compared to Vista SP1's upgrade behavior. Hernandez found that Windows 7's upgrade speed was faster than Windows Vista. Interestingly, the time it took for Windows 7 to upgrade from Windows Vista SP1 outpaced that of a Vista SP1 to Vista SP1 upgrade procedure.

At its very worst the upgrade from Windows Vista to 7 was found to take a whopping 20 hours. At that rate, a user would likely have less downtime if he or she just did a clean install and restored/reinstalled programs.

Most users will average upgrade times lasting just a few hours, but those with slower systems or exceptional amounts of data will have to set aside a good portion of the day just for the automated process. All clean install systems upgraded in around 40 minutes or less.

Source: tomshardware.com