This has not been my experience. I maintain various popular and not-so-popular projects (although maybe not to the level of gnome calendar) and I just work as much as I want. If I don't feel like handling an issue right then, I say I won't have time soon and that PRs are appreciated.

The vast majority of people are understanding, I don't remember anyone making a fuss about it.

Maybe because the consumers of your work are developers? GNOME Calendar (the OP's project) is an app for end users.

Possibly, but not all of them are. This isn't, for example:

https://github.com/skorokithakis/catt

And at most we got a few users asking about why they couldn't install it on 2. Maybe my users are just better-behaved.