If you're a casual Windows Vista user with just a few documents tidily stashed away, upgrading to Windows 7 should take less than an hour. More likely, though, is a three-hour upgrade for heavy users with lots of stashed stuff.
Chris Hernandez at the TechNet blogs ran three types of computers—low-end, mid-range and high-end—through a Windows 7 upgrade with four different configurations each, reflecting the file and application loads of a "clean user" all the way through "super user". A clean user profile on high-end hardware was able to upgrade in about 35 minutes, while a super user on mid-range gear would need about 20 hours to upgrade.
Actual users' experiences will vary, of course, but it's worth checking out your upgrade time estimates to have enough time set aside for an upgrade, or at least consider a clean install. In one Lifehacker editor's experience, a very clean and tidy laptop took about an hour to upgrade from Vista to Windows 7, while a laptop with "an obscene amount of stuff installed" took a good eight hours or so to run through—though both upgraded successfully.
Note also that the timed results were within 5% of the upgrade times for Vista SP1, so it's not exactly breaking news that Microsoft upgrades can take quite some time. But we're guessing quite a few more systems might see Windows 7 discs slipped inside than Vista, so hopefully more users will be prepared.