Anyways, down to my answer. This is based on my 30 years of making and sharing Open Source software. I am open to lectures from nonpractitioners but please recognise that I speak from experience.
I think a project takes a few hours per month to keep running. That's testing and merging Dependabot PRs, triaging bug reports and hopefully fixing a few, checking on security updates and hopefully fixing some of those, too.
Adding functionality or improving performance or reliability takes additional time.
So, I'd budget about 8 hours per month. For me, that means I can only realistically maintain about 1-2, maaaaybe 3 projects at a time, unless I'm getting paid.
Edited 7d ago
I don't think we do Open Source maintainers any favours by hiding these numbers.
It's OK for volunteers to do a job badly, or not to do the job at all. But that doesn't mean the job doesn't take real, measurable time to do well.
Knowing what that time demand is can help them prioritise relative to other parts of their life, or find other ways to get the work done, or accommodate themselves to not doing the work.
Edited 7d ago
Anyway, thanks for the feedback and the replies. I hope for people who are new to the practice or are wondering how they compare to others, the results are helpful. I've definitely found the experience useful.