A new system for moving comments to chat has been implemented. Basically, posts are auto-flagged if there are too many comments, enabling mods to copy all comments to a new chatroom (and then spend some time deleting what doesn't belong on both side [sic]).
While this system is less than what you could hope for in several ways, one thing stands out: "many" is defined to be twenty.
I don't think this is a good threshold for cs.SE; we have far shorter series of comments that I'd like to be able to move to chat, in particuar meta discussions that don't warrant a Meta post.
Shog9 from SE has proposed we collect discussions we would like to (have) move(d) so we can figure out what a good threshold for our site is.