This is a highly variable test: Results may vary.
Steps to Reproduce: Install the operating system, then run its update manager (if it has one).
Gentoo: As this system compiles all software from source, it loses handily (it takes 2-3 hours to update Firefox/Thunderbird, most of it compiling the software). It can take longer than three hours, but that is rare.
Fedora 18: ~15 minutes
Ubuntu 12.10: ~15 minutes
Windows 7: 3 hours (the reboot process added considerably to this process)
Windows 8: N/A (but not expected to be all that long -- yet).
Fedora and Ubuntu take this one away, since they both do not compile all of their binaries from source on the machine they are to be installed on, which Gentoo does. They are both internet-speed related times, hence the ~. Times will vary, based off of internet connections.
Windows 8 cannot be judged yet, as it is not yet available to me.
Windows 7 comes in third, in part due to the length of time it spent configuring updates.
It is important to note that this is somewhat of an apples-to-oranges comparison: Windows only updates the core system files whereas Gentoo/Fedora/Ubuntu update all of the software on a computer.
In update ease of use, Fedora and Ubuntu come in a dead heat (one command, "sudo yum update -y" or "sudo apt-get upgrade -y" although Ubuntu does need an additional command to update the repository information.
Gentoo is slightly more difficult, in that some software cannot be installed at the same time due to conflicts (and the update process will find those, even if they were successfully installed), and it can take awhile to start the update as well.
Windows 7 and Windows 8 will not update non-OS software (although Windows 8 will update software through the Microsoft Store, which most applications are not in).
Therefore if ease of update or update times are important to you, do not use Gentoo, Windows 7, or Windows 8. Again, Windows shouldn't be used in this case becuase every piece of software has to be updated individually whereas Gentoo/Fedora/Ubuntu update all installed software automatically. Gentoo should not be used becuase it takes a long time to compile the binaries for the updates.