Doing a quick straw poll among friends I reckon it's around once every 6000 hours! As 4 Greens says the better equipment you fly, the better off you are! Most people I know have had failures in pistons, although I do know of two total failures in turbines, one was a fuel pump and the other reduction gears...
Jet's of course fail everytime you get in the sim!
Contrary to popular training practices, not many failures happen on take off!
