I've had the same install going since January 2012:

    > head -1 /var/log/pacman.log
    [2012-01-22 14:55] installed filesystem (2011.12-2)

In that time I've converted the install in-place from x86 to x64, migrated from legacy boot to uefi, replaced the entire RAID set twice, motherboards, CPUs, etc. It's my own ship of theseus. Many of these tasks people would say to do a reinstall, but I've always been able to find a guide on the arch wiki to do it in-place without losing anything.