Comments should only be used if they give any constructive input to the post (Q&A) and don't turn out to be a long/open ended debate/discussion.
There is nothing wrong if there is a prolonged conversation with a fellow user, however in such case, it's better to use Chats, as they are meant to serve such purposes and even to talk on off topic stuffs as well.
The whole guideline are present in privileges section of our site. But to concentrate on what explicitly asked here, lets start with comments:
When should I comment?
You should submit a comment if you want to:
When should I comment?
- Request clarification from the author;
- Leave constructive criticism that guides the author in improving the post;
- Add relevant but minor or transient information to a post (e.g. a link to a related question, or an alert to the author that the
question has been updated). *
So its quite clear from this three points. First, comments are fro clarification, I do have tendency to use the comments for the first reason more. Rather then flagging, downvoting and close voting straight its better to ask the questioner for clarification in case of doubt until its non salvageable. Second is constructive criticism, "constructive" is important part of it and we should not forget be nice policy too. Third but not the last is minor or transient information, to add something related in comment.
But its better not to drag the comments for infinite discussions, it will uncomfortable for a new visitor to see 100s of message under a single question, longer discussions should be moved to chat. And chat also give you freedom to discuss unrelated stuff too. But under a question, they might get removed.
Now the second part of the question is
When shouldn't I comment?
Comments are not recommended for any of the following:
Suggesting corrections that don't fundamentally change the meaning of the post; instead, make or suggest an edit;
Answering a question or providing an alternate solution to an existing answer; instead, post an actual answer (or edit to expand an
Compliments which do not add new information ("+1, great answer!"); instead, up-vote it and pay it forward;
Criticisms which do not add anything constructive ("-1, see previous comments you scallywag!"); instead, down-vote (and provide or up-vote
a better answer if appropriate);
Secondary discussion or debating a controversial point; please use chat instead;
Discussion of community behavior or site policies; please use meta instead.
First if you are suggesting an edit, why didn't go forward and do it? That's what the edit privilege is for. Second point is not to give alternate answer in comments, multiple answer can't be a bad idea until you are copy pasting from existing one. No need to talk on third point, "+1" comments are useless and should be removed, if you want to +1, then upvote, give bounties, accept answer if its your question. Next point is about a non constructive comments, no need to provide "same as above comment". Next is controversial topic, controversial topic= lots of lots of comments and it should be done in chat. But remember to be civil while discussing them , don't attract flags and don't make people to raise eyebrow. Last but not the least "community behavior or site policies" and for that meta is the right place not comments. You can refer it in comment like "refer respective meta" with links but no need to discuss them in main site.
For other details refer privilege or help center section or to A guide to moderating comments
On the bottom-line, remember that comments are not permanent and they can be deleted. Refer here for related discussion.
Lets talk about third place now. Chat in stack exchange is also like third place, first is you main site, second meta and then come chat.
It also have a guideline to be followed. But to summarize it, chat is less strict then other two places and we can have fun in chat too. Mods also can be seen in the our main chat-room and you guys can join us and we don't bite.
Chat-room also fall under be nice policy and we should respect our fellow user there too. We can discuss about a Q/A, meta topic, Hinduism or even off topic stuff. We can discuss about our doubts too, chats are good way to contact mods too. And in chat there is no limit to lengthy discussion, just don''t annoy others and respect there opinion too and if it turns offensive, ping a mod and leave it to them to handle.
Here is a guideline to create new chat-room, but remember individual rooms didn't attract much audience and those room can be freeze and deleted automatically for inactivity. Only create them when they are needed, else no need to brag if they get freeze or deleted, mod can unfreeze and undelete but it will be temporary and activity is need to keep a chat-room alive.