In my experience on this site, over time, plot points in a games storyline become unnecessary to use the spoiler tags (I myself have done this, removing spoiler tags from answers after a period of time - in one case this was after 12 months, or more).
Do we have a recommended timeframe before spoiler tags are no longer necessary?
You're misunderstanding the point of spoiler tags. It does not matter how old a game is (or a movie, book, TV show or other form of narrative art). Putting a deadline on how old something needs to be before it no longer needs spoiler tags is nonsensical if you think about it. It means that anyone who discovers the artwork after that period, be it through age, social restriction, embargoes or just ignorance, is essentially told to deal with it.
People are still discovering Star Wars for the first time. Some people are only now finding out about the Harry Potter novels. And there are definitely people out there that haven't yet had the chance to play Spec Ops: The Line. Depriving people from the fun of experiencing a major plot twist in the way the creators intended, simply because they weren't available at the time the narrative artwork was current, is not a nice thing to do.
Spoiler tags should in my opinion be used for timeless, yet unexpected story events that completely throw a story around, or where knowing the events in advance makes later events less impactful. To take an example from Batman: Arkham Knight, it's fine to spoil that there's a fight with the Arkham Knight, or that the main villain is the Scarecrow, or that Barbara Gordon is in the game, because those are all explicitly mentioned in press releases, reviews, the official site for the game. However, some more unexpected events, like (in case it's not obvious: spoilers for Arkham Knight)
the identity of the Arkham Knight, the appearance of the Joker or Poison Ivy being an ally
are major story events, especially for those that have played the earlier installments in the series. People playing the game don't expect these things, and knowing their existence in advance ruins some of the enjoyment from the game.
To give another example, purely theoretical: Suppose the title of the game is "The general betrayed us", or that the store page mentions "After General Hugblankie bombed his own troops deliberately, Captain Cuddlesnout teams up with Major Snugglebunny to bring him to justice". in this case, if it's actually the general that betrayed you, then that shouldn't be in spoiler tags, because it's clear from official material. However, if either of the other 2 characters is the traitor and the store description or game title are deliberately misleading the player, then that's a major spoiler that shouldn't be put in clear text, no matter whether the game is released in 2 days or was released 20 years ago.
Note that both questions and answers should be completely valid as such if the reader doesn't read what's behind the spoilers. As a rule of thumb, if you can delete the spoiled text from the post and the post still makes sense, it's fine. If the spoiled text is needed to understand the post, you should try and narrow down the spoiler until the post is understandable, or even leave the spoiler away entirely.
Finally, a case can be made for using spoilers to hide solutions to problems the player has to solve while playing the game. Again, knowing these solutions in advance ruins the enjoyment of the game. If the question explicitly asks for the solution, then you don't need to use spoiler tags. If the solution is needed to give context to the question, but the question can be made understandable without the spoiler text, then it's okay to use spoiler tags.
