The timescale of human evolution to date is on the order of 2 million years or so since we branched from our common ancestor with chimpanzees.
Somewhere around 80,000 years ago (plus or minus) we seem to have reached modern levels of cognitive ability, specifically, the capacity to keep 7 +/- 2 things in mind at the same time, as evidenced by the anthropological record -- stone artifacts and such.
Written history and most civilization spans roughly 6,000 years.
So, if you're looking at what "sustainable" might mean, I'd argue for somewhere within that timeframe. A minimum of 6,000 years, a reasonable upper bound of about 2 million.
