A "thread" is a series of related posts. Since lots of people come in cold to this forum, the threads are helpful because they keep conversations on the same track. It becomes confusing if someone starts a new thread when intending to respond to an existing post.
As nearly as I can tell, there is no timeout period for threads- note that just this month someone posted to one that was several years old. You can go back as many months as you want to find a post for resurrection. For example, suppose that someone posted a question about a Zenith 12-S-266 dial belt back in 2006. Using Google and the keywords "12-S-266", "Dial belt", and "Nostalgiaair forum" you should be able to find the original thread. Then you can revive it in the current month by "replying" to it.
If you get no response to a thread that you initiate, you can always reply to your own post, in which case the original post will be "promoted" to the top of the thread stack for a little extra visibility.
I don't think that anyone wants to be a "thread Nazi" about this- it's a little unreasonable to think that just because there was a thread about, say, Philco photofinish restoration back in 2007, a current poster who seeks that info would be obligated to first find that post and add to it rather than starting a new thread on the same topic. It's not necessary or useful to do a "patent search" every time you want to broach a new-to-you topic. But starting new threads on the same topic in the same month is the other extreme.