800 years. Eight hundred years is good shelf life for a book.
The Black Book of Carmarthen is the oldest manuscript written in Welsh. It dates about the 1260s time frame. It passed through innumerable hands before it was at last deposited in safe keeping in the University of Cambridge (makes you wonder how many were lost on the way).
It is filled with verse, some of it about Arthur and his crew. Long studied it has proven a hall mark in early English literature. However, its role just got a little bigger. Researchers have found traces of older text, written in the margins. It seems that someone, somewhere along the way, erased huge chunks of it.
They are only just now being revealed.