I voted for all categories because I think that after a long enough time without new contributions (e.g. 6 moths, just to exemplify), even topics in the most relevant categories could be safely closed.
However, I recognize that it could be tricky to set a specific time that suit every topic in every category…
Sometimes number of replies is more relevant than Category itself (e.g. a topic with <5 replies can be probably be automatically closed after 4-6 weeks without anyone even noticing, whereas a topic with 100+ replies should probably stay open for a bit longer.)
Would it be feasible to have different “topic lifespans” based on the categories and/or number of replies?
If it is not, and going full manual is too much of hassle and time-consuming, I would set a reasonable long time and live in peace.
Finally, pretty obvious, I would leave a “manual backdoor” for Community Managers (1.0, 2.0, and 3.1 USB-C ) to leave open or close specific topics, in spite of any set automatic rule.