There is no clear line. Some factors:
How old is the original thread? A few months is OK, a few years, no.
How long was it? Don't trick people into reading a lot of outdated ideas
...because the last few posts interested you.
how rigorous was the thread? Just chatter? Start something better.
Who was participating? People who rarely posted since then? Forget it.
If you value some of the original, start your own, but post a link to it. That way, it only "brings it back" for those interested.