According to this piece of news, Windows 7 upgrade from Vista can take a almost a day !

Whoa ! I was tempted to get a Vista system with upgrade option but now I would not.
Slashdot Technology Story | Windows 7 Upgrade Can Take Nearly a Day

A clean 32-bit install on what Microsoft calls ‘high-end hardware’ should take only 30 minutes. In the worst case scenario, the process will take about 1220 minutes. That second extreme is not a typo: Microsoft really did time an upgrade that took 20 hours and 20 minutes.