History in 'A Song of Ice and Fire'

There's a fascinating post over at Tor.com which examines the history of the realms in G.R.R. Martin's A Song of Ice and Fire:
It’s not so much that A Song of Ice and Fire lacks history...just that there’s toomuch, to their thinking. After all, the Long Night is dated to some 8,000 years ago, the wars between Valyria and Old Ghis to 6,000 years ago, and there are other events noted down as happening many millenia before. In thesecond episode of the TV series, Ned Stark informs his bastard son Jon Snow that Starks have been manning the Wall for thousands of years. There’s two difficult ideas buried in that statement.
First, an organization that’s been doing something for thousands of years (the Roman Catholic Church has nothing on the Night’s Watch). Secondly, a family that’s existed for thousands of years (the Imperial Family of Japan is about 5,500 years younger than the Starks claim to be). This sort of thing simply doesn’t happen in our world. It’s literally incomprehensible in any realistic sense. These vast time scales are comprehensible to us in the real world only because of the development of modern archeology over the last couple of centuries. To people in the Seven Kingdoms and on Essos, there really should be no knowledge, much less understanding, of such time scales....

As I struggle to locate my own writing project inside a solid and authentic mounting I find myself thinking  very hard about how much history to give my world; how thick should it be?  What about different languages?  How much of these Elder Races should be left in the world?  I don't yet know what the magic amount is and I suspect I'll only learn that mathematical equation after input from my beta readers.