I was wondering the same thing when I started flying turbines in 1980. As close as I could get was that Allison claimed that the rate of "early removal" on their 250 series engines was one in 50,000 flight hours. While it doesn't answer the question directly, it says something about reliability.
There are many reasons for an engine failure, from a loose fitting on a fuel control to sloppy refueling practices. The best defense against engine failure is a good preflight inspection. A wise instructor of mine once told me that a helicopter will almost always warn you before it lets you down. (Ok, he didn't say 'Almost', but I had to throw that in for credability). It's up to you whether to heed or ignore the warning signs.