No real time constraint, I've seen highly discussed and upvoted material that has been 10, 20, even 50+ years old.
What's appreciated is a time context - the mods will often add a date to the title of active submissions if they're from "the past" (which is subjective - sometimes six months old is "ancient" in a fast evolving domain, some 50 year old essays have a 'timeless' quality that is relevant in the now).
