Here's what an unofficial essay on Wikipedia says and I think it sums it up sensibly:

> The copied material should not comprise a substantial portion of the work being quoted, and a longer quotation should not be used where a shorter quotation would express the same information. What constitutes a substantial portion depends on many factors, such as the length of the original work and how central the quoted text is to that work.

https://en.wikipedia.org/wiki/Wikipedia:Quotations#Copyright...