So question, what is considered too old for a thread to continue discussion in and consider it a "necro"? Is it a rule made by each moderator? Is it due to popularity of the discussion? Size of the forums you are posting in? Is there really anything wrong with continuing a conversation in an existing thread that may have been pushed a few pages in rather than clutter up the forum database with more threads on a similar subject?
In some cases you get the whole "there is already a thread on this, use the search" from people if you post another topic; where again you are breaking these rules of the forum.
case in point this thread
Was that thread too old to continue discussion in? I don't think the person who did it had any intention on trolling anyone with revisiting the conversion. I was going to post in it, but it was of course locked. I could start a new post on this topic, but again, I'm probably breaking some forums rule in doing so.
So my suggestion would be just to have threads auto lock after they drop off 50 pages back AND hit a one year date. If the forum only has 2 pages well then it can't be considered a necro, it's an ongoing discussion. This way we know ---ok, maybe we should just start another discussion thread on this issue and it's not looking like we are creating numerous threads on the same topic because we are scared we are gonna get a warning for necroing.