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If you plan to upgrade from Windows Vista to Windows 7, you might as well plan on doing nothing else for a day. Preliminary reports indicate that some Windows 7 upgrades can take as long as 20 hours. If you’re a heavy Microsoft user with a large hard drive and with many, many applications, stock up on coffee, pizza or beer. Or just have one of those upgrade parties with your friends.

ZDnet reports:

Over the weekend, several leading tech websites zeroed in on one alarming snippet of data that Microsoft tester Chris Hernandez published on his Technet blog: With enough data, on sufficiently underpowered hardware, Microsoft found that a Windows 7 upgrade from Windows Vista SP1 can take as long as 1214.86 minutes, which is (hang on while I fire up Calculator here) … 20 hours, 14 minutes, 52 seconds. Give or take a few milliseconds.

Ars Technica also reported:

The biggest thing that stands out about this chart is the very broad range of the upgrade time: from 30 minutes to 1,220 minutes. That second extreme is not a typo: Microsoft really did time an upgrade that took 20 hours and 20 minutes. That’s with 650GB of data, 40 applications, on mid-end hardware, and during a 32-bit upgrade. We don’t even want to know how long it would take if Microsoft had bothered doing the same test with low-end hardware.

Recommendation:
If you can, you are better off backing up your data and installing a fresh install of Windows 7. Total upgrade time: ~30 minutes.