I usually do updates about once a week. If I need to install a new package, I'll usually refresh everything then as well. But occasionally it's longer than that

I usually do updates about once a week. If I need to install a new package, I'll usually refresh everything then as well. But occasionally it's longer than that. I just forget for a week or so, go on holiday, etc. Most of the time, I come back, do an "Syu", and it's fine. Occasionally though, things go pear-shaped. Once or twice I've needed to manually fetch packages from one of the archive sites and manually copy it into pacman's cache directory to get things up and going again.