There is some tension between different parts fo the documentation here on Meta, the help docs, user comments and voting behaviour.
One banner says: "We are looking for long answers", and additionally we look for sources, proper attribution and quotes – instead of plagiarism – and within context for links:
Links to external resources are encouraged, but please add context around the link so your fellow users will have some idea what it is and why it’s there. Always quote the most relevant part of an important link, in case the target site is unreachable or goes permanently offline.
All that so far reads somewhat like the "longer the better", and SE has a limit on characters allowed for any answer so there is a technical absolut limit anyway.
But then we have all short attention spans, and on SE especially it seems quite important to hit the sweet spot for answer length.
While I noticed that some users seem to think that writing just one sentence without sources should be enough, others think that mass is a quality on its own and write really long answers, almost by default.
What I do not like is that there seems to be quite an influence on the perceived quality of a post –– based on length alone. That is users judging long posts automatically as good or the opposite: stopping right after tl;dr treating that abbreviation as an order.
As a personal observation I estimate that my long answers do much better than my short ones, vote-wise, but the long ones get criticised for being long. Shorter ones might get challenged for uncited sources, links are not followed up or read and then some comments come in questioning a summary or paraphrase about what is clearly behind that link. That is a bit confusing.
So, what are the guidelines for estimating the optimal answer length?
How should someone writing an answer orient himself on the following factors: estimated reading time, scrolling avoidance, number of links, content around links, length of quotes and context, number of quotes?
Are there even more factors to keep an eye on?
"As long as is strictly necessary" seems like a wobbly response that leads back to the first character of this post. Being parsimonious is a virtue but this is mainly about additional heuristics a writer would be advised to observe.
My guess is that more than one answer representing different "user styles" will be necessary for this.