Let's say you're assembling a background for a science fiction setting, whether it be in the tradition of space operas of old or informed by more modern tropes and subversions of tropes, which involves interstellar or even just interplanetary travel. There are a great many concepts that could be plugged into this setting - the Brave Space Patrol keeping the rule of law strong on the spacelanes, the Wretched Hives haunted by smugglers, criminals, and ne'er-do-wells, the Free Traders who flit from world to world in search of wealth and who frequently find adventure, to name three.
One aspect of the background that many worldbuilders can't resist dropping in goes beyond these - the Precursors. Though they're rarely called that explicitly, a setting's Precursors are usually a great and powerful species that rose to vast heights of power and dominance over the setting's Known Worlds, only to inexplicably disappear in a manner that leaves crumbling monuments and inscrutable relics for intrepid explorers to puzzle over. Whether or not they're complete assholes or just lazy bastards is up to the writer. Frequently they're the source of whatever superscience is present in the setting, allowing more fathomable people to go across the stars without needing to go through the million-year correspondence course to get the qualification to build the starways.
There, though, I exaggerate. One of the problems I find with Precursors in a great many works is simple - they're not ancient, not really. Plenty of "ancient, mystical, powerful" and yet vanished empires populate science fiction, but most of them are ancient only in the timescale of human civilization, and not in the cosmic timescale.
The closest concept of the Precursors to the mainstream today may be the Forerunners from the Halo series. They built the ringworlds and a galaxy-spanning empire, yes - a hundred thousand years ago. The same is true of the Precursors themselves from Star Control II, who are described as having vanished "a thousand centuries ago" - in other words, one hundred thousand years ago. Mass Effect's Protheans disappeared a mere fifty thousand years ago. Star Trek contains multiple Precursors, though some avert this proximity in time - the Guardian of Forever, recall, is older than Sol. Nevertheless, it seems to me that Precursors will frequently be placed in the "recent distant past" - a hundred thousand years appears to be a significant psychological cutoff point. The Ancients in Descent: FreeSpace push this even closer - having only disappeared eight thousand years ago, they were in fact coterminous with the beginnings of human settlement, though the Shivans do predate them significantly.
Written works, on the other hand, seem to subvert this more frequently. The builders of the clanking replicator machines that became the Taloids in James P. Hogan's Code of the Lifemaker were active 1.1 million years ago. In S.M. Stirling's Lords of Creation series, the titular Lords terraformed Venus and Mars on the order of, if I recall correctly, two hundred million years ago; certainly more than sixty-five million, as a saurian ecology was preserved on the habitable Venus. Larry Niven went even further back in the background to Known Space, with the ancient empire of the Thrintun having dominated the galaxy more than a billion years ago.
I hope that it's just a case of selection bias here, because there's no reason there should be such a disconnect in terms of the ages of various Precursors. The universe is vast, and has been around for literal aeons - personally, I find the concept of artifacts and ruins that have survived across the endless ages, in space or on airless worlds where there is no weathering as on Earth, far more awe-inspiring.
POSTSCRIPT, 11:37 AM: James Nicoll also has a post dealing with subject, as it turns out. I didn't see it before I wrote this. What a synchronicity.