Sarah Rhodes has published "Breaking Down Link Rot: The Chesapeake Project Legal Information Archive's Examination of URL Stability" in LLRX.com.
Here's an excerpt:
In analyzing a single sample of these original URLs at annual intervals, the prevalence of link rot was 8.3% in 2008, within zero to twelve months of the content being harvested. One year later, twelve to twenty-four months after the content was harvested, link rot in the same sample was found to have jumped to 14.3%. In the most recent analysis, in 2010, link rot was found to be 27.9%. In other words, link rot increased from about one in every twelve archived titles in 2008, to one in every seven titles in 2009, and finally to about one in every 3.5 titles in 2010.