Jonathan Chance just closed a thread on how do we solve the opioid crisis because there had been threads on this topic 4 months ago and 2 years ago (and others before that):
http://boards.straightdope.com/sdmb/showthread.php?t=811356
To me 4 months seems to be plenty of time to repeat a topic.
It’s worth mentioning they were made by the same OP.
Who has been warned not to start threads on duplicate topics, IIRC.
But I applaud this OP’s username and thread topic combo. 
My understanding is it’s not a zombie thread until 6 months have elapsed.
There is no hard and fast rule about what makes a ‘zombie’ thread. Such things are very context dependent and therefore hard to define.
In the case PastTense is asking about the OP of the thread in GD is in the habit of starting many threads. This, in itself, is not a problem. However, before I initially intervened, he was in the habit of starting threads and not returning to them. I asked him to begin participating in threads he starts.
The idea that he is asking pretty much the same question in threads just a few months apart becomes again a symptom that he’s not actually looking for debate and discussion but rather starting threads simply to start them. I admit, the older thread from 2014 would not have set off my alarm except that there was a thread within the last four month on an identical topic.
Again, all things are context-dependent. Had the OP not had the habit of starting non-responsive threads and then developed the habit of repeating threads it might have been OK. Especially if the new thread acknowledged the old thread and mentioned what made the new thread different.