Usually, timeless discussion threads are always relevant, as the age of the thread doesn’t matter. However, that doesn’t mean you can bump multiple old threads in one day or bump a thread last active when Obama was our president. However, the threads that always get locked once they’re bumped from the dead were the threads last active before ACNL’s release. Those I definitely don’t recommend bumping.
So to answer your question, it isn’t about how long it’s been since the last time someone posted. It’s when was the last time someone posted.