The answer is November 23rd, 2019.
Meaning? Meaning that I installed my current desktop operating system almost exactly a year ago. Old-time readers of mine will be astonished, I think, that I've managed to maintain the same operating system installation for anywhere near that long! Past practice was for a fresh install to last a month or two at most before being replaced by the new shiny (whatever it happened to be!)
So lasting a year is real testimony to Manjaro's qualities (for that's my current distro of choice, using the Plasma (KDE) desktop environment. It has worked well and has enabled me to run everything I've needed it to, without fuss or hinderance.
Except... except... There are one or two signs of 'decrepitude' creeping in. It's inevitable after a while, no matter what your O/S is: you install things, which install dependencies, which you then remove, only some of the dependencies get left behind, and they don't play so well with whatever new thing you've installed and so on. It happens -and it's happening on my desktop, but not in terribly obvious ways, nor with particularly disastrous results. Occasionally my media player will stop playing to my sound card for "reasons". A quick switch in the Audio system settings from the current device to another and then back again fixes things, and music plays once more until it flips out again a week or two later. It's slightly annoying, but it's not a show-stopper. The fact that if I run Audacity (a sound editor), all audio immediately stops for all apps (including Audacity) and only works again if I do the audio-device-switcheroo once more is a bit more annoying, but then I don't use Audacity too often anyway. So again, I could live with it. There are lots of other, similar, minor niggles that are common, but not dramatically awful (the one where a down-scroll of my mousewheel results in an up-scroll of the page I'm looking at is perhaps the worst and is a constant annoyance).
So, given the 1 year anniversary approaches, I think it time to see if a different distro and a fresh O/S installation help these minor matters whilst maybe just giving me the pleasure of "it's new!" for a time, too! My new distro of choice will therefore be
Fedora 33 Workstation, which was newly-released last month. It's going to be a Gnome-based distro, which is a substantial change from my normal KDE preference -but I figured it would be worthwhile trying to stick to Fedora's 'native' desktop to keep the installation as clean as possible. We shall see how it goes... but I'm going to wait until the 24th November before going ahead, because I've got terabytes of data to backup first!
Edited to add: Well, that didn't last long! A test install of Fedora 33 I did on a spare laptop quickly revealed that it was practically impossible to install Emby Media Server on it. Not that this is Fedora's fault, particularly: look at the singularly Ubuntu-esque download instructions for 'Emby on Linux'! There are workarounds, but the ones findable on the Internet tend to relate to much older versions of Fedora and when applied to Fedora 33, I simply got a lot of 'can't install this package because... this dependency has no installation candidates...' and such like messages. On Ubuntu 20.10, it's an almost one-click affair to install it... so I'll be going with that distro in about a week's time after all!