I don't think anyone has researched this.
Going around and looking for people who had there cards fried by intensive usage would be the best guess...
If someone actually wanted to test this, it would require tens of GPUs (bought with cash from their own pocket).
After that multiple PC's would have to be run thousands of hours with and without stress just to get a vague idea.
There was an interesting article on E-sports which 'accidentally' added a miner to their software.
Read the article, but keep in mind a lot of these computers were not prepared properely for mining!
http://tech.slashdot.org/story/13/05/01/217230/e-sports-league-stuffed-bitcoin-mining-code-inside-client-software