I recognize this will vary depending on how much you self-host, so I’m curious about the range of experiences from the few self-hosted things to the many self-hosted things.
Also how might you compare it to other maintenance of your other online systems (e.g. personal computer/phone/etc.)?
It’s as much or as little as you want to. If you don’t want to change anything, you can use something like debian and only maintain once every 5 years (and you could even skip that).
I personally spend a little more, by choice, because I use gentoo. But if I’m busy, I can avoid maintenance by only running routine updates every couple of weeks or so.