<A target="_blank" href="http://arstechnica.com/microsoft/news/2009/09/microsoft-upgrade-to-windows-7-can-take-up-to-a-day.ars">Microsoft has tested various scenarios</A> for upgrading to Windows 7. For example: a clean install should take about 30 minutes. But. If you're upgrading, Microsoft's tests show it could take as long as 20 hours.