That's a tricky question to pin down. Originally the Russian engines had a 500 hour TBO, and could be rebuilt a certain number of times... but these limits were determined based on military use. I couldn't find anything quickly on the net, but I believe the engines currently available in the US are rated for 1500 hour TBO. I'm sure you could get an accurate and detailed answer to this question from Gesoco Industries at
http://www.gesoco.com. Interestingly there is a version of this motor used to drive generator units, and at 70% power they have a life expectancy of over 20,000 hours!
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