I’ve noticed for quite some time that some topics will close in 14 days (a year, ten years, etc.) after the last reply. And then there are topics that don’t list any closing period. I couldn’t find the answer to this anywhere…
So what determines when a topic will close or not at all?
Just curious…
Discourse forum admins can set different topic-closing times for different categories. On this forum, for example, it looks like topics in the “Reading” category auto-close after 10 years by default, and topics in the “API and third-party apps” category don’t auto-close at all.
Mods can also override the setting for an individual topic (which is sometimes used to prevent argumentative topics being rehashed forever and ever). I don’t know whether the mods on this forum ever use this ability.
Afaik, normally, a topic would after close a year after last reply.
But in some sections the limit is different.
A topic would also close once it reaches 10 000 posts (a continuation topic would be auto-created).
Also, a topic temporarily auto-closes if there are too many flags in it.
Also, a topic could be closed by admins either because it was inappropriate or because the author asked them for it directly.
There might be other cases of which I don’t know…
Thanks to @mbrubeck and @trunklayer for your clarifications!
I guess this is similar to Japanese grammatical rules that say you use X, if you do Y, but Z if you don’t, unless it’s the third Wednesday of a Leap Year, then all bets are off! 
The reading category is special cased to not close after the usual year for the benefit of threads for book clubs and the like where it’s nice if people reading the book after a club has long finished can still ask questions.