I like this part of the Ubuntu code of conduct:

> Step down considerately

> When somebody leaves or disengages from the project, we ask that they do so in a way that minimises disruption to the project. They should tell people they are leaving and take the proper steps to ensure that others can pick up where they left off.

I don't think a maintainer owes anybody free labour, but I do think a good maintainer will hand over the keys to someone else when the needs of the community require it.