No, we neither do nor should have such a rule.
Spoilers on this site don't have a shelf life per se. We have a standing and practiced policy to avoid spoilers in question titles but there is theoretically no temporal factor involved in this policy (I'll elaborate later why I said "theoretically"). Introducing such a time factor comes with a variety of problems:
- First of all, how to choose a reasonable time frame for something like this? How long is enough time to wait until the majority will have seen it? What even is a "majority"? And how known is "commonly known"?
- You already bring up the problem of enforcing this. Yes, we currently are subject to the mercy and effort of the community to care for proper spoiler-less titles. But you're bringing an additional factor into this by having people regularly check old questions after a certain time-frame in order to improve titles.
- It discourages people to write sensible titles. Rather than that we will just get lazy titles because we expect them to be improved later anyway (again, by whom?). But I'd rather not that we're stuck with bullshit like "Why did this character do that?" for months until someone has the mercy to fix it. Rather than that I'd prefer we have reasonable titles from the getgo.
- On the other hand it encourages people to spoil away the entire film in the title just because their 6 months (or whatever) are finally over.
Instead of this I'd rather people put effort into writing a proper, sensible and reasonably spoilerless title right now rather than posponing this effort for months. Yes, this is all quite fuzzy and I can see how pointing to a hard and fast rule and saying "the film's 2 years old, spoil away!" can be easy, but we don't need to rule-lawyer all common sense away with some arbitrarily chosen time period (and any time frame will be entirely arbitrary).
With all that said, there is a certain indirect time factor involved in all those spoiler issues in practice, especially with very pop-culutrally hyped new releases (and I'm sure it's no coincidence that you're asking this question during Avengers: Infinity War season). There are various reasons for this:
- With new releases the chance is of course much higher for people to not have seen the film and thus have it spoiled by an all too revealing title. The more time pases, the more people see the film, and the less of a practical problem a spoiler in a title becomes. There's also spoilers that, while once being big things have seeped into common knowledge. Anyone who doesn't yet know who Luke Skywalker's father is likely doesn't care about it anyway.
- Especially with such big blockbusters there tends to be a significant degree of "spoiler paranoia" right after its release on the internet at large. This isn't always entirely reasonable and people will take umbrage to rather innocuous titles simply because it mentions a character name or things like that. However, over time (and often quite shortly) this general spoiler paranoia will even out into a more reasonable approach, especially since the loudest of those voices are likely to go see the movie as fast as possible anyway.
- Related to the above problem is also that questions about such films have a big chance to land on the HNQ list, which is still one the major sources of spoiler complaints. Once the immediate hotness of the film is over, this tends to ebb out, too (until the next GoT season or Star Wars sequel, that is).
These factors might very well lead to us maybe wanting to take a little more care especially with new releases of highly anticipated films. But it does not mean we ought to give in to the, frankly out of control, spoiler paranoia that plagues the internet and obfuscate our titles away into incomprehensibility. It might as well mean that you could get away with a spoiling title some time after the movie is out simply because noone complains/cares. We can't always police everything and if noone complains, it might just not be a practical problem. But this doesn't free you from the responsbility to be reasonably mindful with your question title. And if someone comes around and complains about a major spoiler in a title, saying "gheez, the movie's 3 years old!" is not an argument we're going to have.
So while there is a little timing involved in this in practice, you see that it's all quite fuzzy and insubstantial. But such is the case with something as subjective as a spoiler. In the same way you can't strictly define what a spoiler is, you can't define when it stops being one either. Its practical impact might lessen over time, but it stays a spoiler to some degree. So rather than making some arbitrary strict rule, I'd prefer we keep doing what we always did:
Be mindful of spoilers, while still writing sensible titles!