I have several virtual machines here with Arch that I often don’t use for months. And when I do use them, I proceed as I do with every update. So before an update, I check if something has been published at archlinux.org/news/ that affects the installation in question. This is done automatically with the help of the tool informant. If something has been published that affects my installations, I take that into account. Otherwise I run pacman -Syu as usual. And that’s it.