From the Intercept writeup:

Rand’s report is based on unprecedented access to a database of zero days from a company that sells them to governments and other customers on the “gray market.” The collection contains about 200 entries — about the same number of zero days some experts believe the government to have. Rand found that the exploits had an average lifespan of 6.9 years before the vulnerability each targeted was disclosed to the software maker to be fixed, or before the vendor made upgrades to the code that unwittingly eliminated the security hole. https://theintercept.com/2017/03/10/government-zero-days-7-years/

Zero Days, Thousands of Nights The Life and Times of Zero-Day Vulnerabilities and Their Exploits

by Lillian Ablon, Timothy Bogart

Zero-day vulnerabilities — software vulnerabilities for which no patch or fix has been publicly released — and their exploits are useful in cyber operations — whether by criminals, militaries, or governments — as well as in defensive and academic settings.

This report provides findings from real-world zero-day vulnerability and exploit data that could augment conventional proxy examples and expert opinion, complement current efforts to create a framework for deciding whether to disclose or retain a cache of zero-day vulnerabilities and exploits, inform ongoing policy debates regarding stockpiling and vulnerability disclosure, and add extra context for those examining the implications and resulting liability of attacks and data breaches for U.S. consumers, companies, insurers, and for the civil justice system broadly.

The authors provide insights about the zero-day vulnerability research and exploit development industry; give information on what proportion of zero-day vulnerabilities are alive (undisclosed), dead (known), or somewhere in between; and establish some baseline metrics regarding the average lifespan of zero-day vulnerabilities, the likelihood of another party discovering a vulnerability within a given time period, and the time and costs involved in developing an exploit for a zero-day vulnerability. Key Findings

Rand (PDF report link on right sidebar): http://www.rand.org/pubs/research_reports/RR1751.html