Recently I managed to find a few topics I thought worthy of starting threads about. I felt a sense of ownership, and hoped my children would do well - strike a chord, excite some good conversation. I suspect many (maybe all) of us feel similarly. As it happens, several of them seemed to have ‘expiration dates’, such as the current DNC thread in Elections. I noticed I felt more benignly about digression as the expiration date neared on the threads I had launched than I did in the earlier stages of the thread.
I got to wondering if Mods tend to be more lenient with devolvement away from the OP in these ‘aging’ threads, or feel a need to maintain the same consistency of moderating to stay on topic as you might with a more robust one?
Well, in at least one situation, the answer is clearly stated in the rules: In a new General Questions thread, posters are strongly (but not absolutely) encouraged to stay strictly on-topic for at least as long as it takes for the OP to be fairly thoroughly answered – during which time, side-tangents, jokes, snark, etc., are discouraged. Once the OP has been reasonably well answered, then digressions are more tolerated.
In actual practice, it’s apparent that this goes on as well in many threads on all of the forums, ISTM.
There are no idle questions just idle posters.
That’s why God invented timeouts.
and Idle Thoughts.
Today is my day off, and I was avoiding chores.
How could you tell?
Yep. Compartmentalising things with a “not to be discussed after” date is just ludicrous. Heaven forfend that they might ever become ‘current’ again…
What a bizarre comment. There’s no such person/thing as ‘God’ and to think, if he existed, that he might write rules for a web forum’s “discussion to be ended by” date is just tragic.
What is DNC?
And what does “forfend” mean?
DNC = Democratic National Convention
Forfend = “Avert, keep away, or prevent (something evil or unpleasant)”
Thank you Gagundathar.
DNC are the backup singers (now recording on their own) of the Hip-Hop group Run-DNC. Their names are Darnell, Nathan, and Chris.
Now, that’s more of the response I expected 
The Unwritten Rule in this case is to remain serious if the question is reasonable. If, however, it is significantly towards the silly part of the spectrum, snark may begin almost immediately.
You just whooshed yourself. Ah, July 2012. I heart you.
That was kinda funny. Once again I must lament the untapped energy in that tremendous woosh. Perhaps FTB can be fitted with some sort of Cranium Mounted Wind Powered Generator.
I love it when a new poster (especially from the erstwhile Summer of 2012 of our discontent)wades into a thread with authority, trying to smack down longtime members while completely failing to understand the nature of how this board works with its memes, snark, double-entendres, riffing between members, etc. It brings a tear to my jaded eye.
On the other hand, he does seem a natural for this place. 
To actually answer the OP:
Yeah, as a thread moves on, there will always be a certain amount of drift as points are raised and those points are questioned or challenged, with the answers to the questions and the challenges raising further points. A thread that is not filled with hostility that gets to page 7 might be pretty far from the OP without getting any serious Moderating attention. On the other hand, if the second post looks to start an immediate hijack away from the OP, it will be addressed more sternly by the staff.
(If the thread does drift into open hostility and is far removed from the topic of the OP, it will often be closed as too much hassle for the staff to babysit, since, by then, it is typically a tiny number of posters just fighting among themselves for the sheer fun of being nasty and there is little chance the original topic will be reviewed, again.)
****Thanks, Tomndebb for satisfying my idle curiosity. For the rest of you, that’s been fun, too!