Time and the New Sun
It's harder than it seems at first glance to decide on the time scale of Gene Wolfe's "New Sun" books. Most of the changes that suggest millions of years - resource depletion, evolution of new species, even the extinguishing of Earth's fires, could have been achieved in far less time through technological intervention.
Trying a simpler question, as a means of getting a handle on the harder one, how much time passed between Ymar's day and Severian's? The clues conflict. Ymar lived in the first days of the dying of the sun, in the last days of Earth's diastrophism, and in the last days of Earth's interstellar empire. (It may have even been in his day that the Autarchy lost most of Earth outside South America.) On the other hand, he lived in a world little different from Severian's, where even the small villages were the same. We also note that it was reasonable to expect a coin from Ymar's time to survive, badly corroded, in Severian's: there is reason to suspect that common words such as 'copper', 'stone', 'wood', often refer to far more durable materials than we associate with them.
Dani Zweig