Ten Thousand Years
We seem to have this huge fear of humanity collectively forgetting things, this idea that has seeped into so much fiction as well, but is it really founded?Even before digital storage and the like, is there really any relevant information that has been lost in the past 150 years? And as to the whole "language barrier" thing, we figured out hieroglyphs with a much smaller corpus.Hell, we know about the meaning of loads of  sites from over a thousand years ago.I feel like we'll have ways of knowing what the site contains forever.