One of my pet peeves in epic fantasy is the way so often things seem to have been going on relatively unchanged for centuries, even millenia. I recently even saw a blurb about a fantasy work I haven't read that mentioned how something had lasted for 50k years. That's for us the year 48,000 BC...am I honestly supposed to believe anything would be accurately remembered from that long ago? That any dynasty or people or institution could have lasted that long with the same identity (and still end up in your standard pseudo-medieval culture)? We're talking something like 8 times as long as the amount of time between us today and the earliest Mesopotamian writings. And I don't know about this case, but too often it seems that the people of the earlier times lived pretty much the same pseudo-medieval-European lifestyles as those at the end.
Even a lesser time frame can be as ridiculous. George Martin, much as I love his Song of Ice & Fire series (before I discovered that I'd completely given up on recent epic fantasy and was only reading older stuff within the fantasy genre and newer stuff outside it), my recollection is of a 4,000-year span for a fairly stable identity to some of the kingdoms that became part of the Seven Kingdoms. And Bakker's excellent Prince of Nothing series has two 2,000-year spans, though in that case one of the things I like is that most of the people doubt the things Achamion and his school are able to remember because of their magic. (Clearly by mentioning those two, it should be clear that this isn't a deal-breaker by any means.)
But let's break it down even smaller. At various times I've researched different parts of the world and the succession of dynasties, kingdoms, ruling powers. India, SE Asia, the Americas. And it's struck me that often stable, influential powers last 200 years at the top of their region, and maybe double that or a little more to include the time leading up to their ascension and then what's left after they fall. So often what was a major city becomes an empty area (especially if weather patterns or water resources change), though of course it's more frequent for a subsequent power to build their own city on the same site if it remains a strategic location. I think we get fooled by how long-standing certain countries and national identities seem to be and forget those that are quickly supplanted.
Of course, I've been chided for the reverse--a crit partner from UK says it's a typical American thing to err in the other direction, giving a much shorter time span, shorter history and cultural memory. I mean, around here a 100-year-old house is something to point out, to tour when out-of-town guests come. We don't have tangible evidence of the centuries of history that predates such settlers, except as artifacts in the ground. In my first novel manuscript he commented on the fact that something which took place only 200 or 300 years earlier had been completely subsumed by its myth, the actual situation forgotten. Now in part that was intentional, as the winners enforced a certain way of remembering it. But it could be that I didn't give enough credit to the cultural memory or those who'd been on the winning side but not themselves rulers.
Well, my writing tends to flirt at most on the edge of epic fantasy anyway.