As I mentioned in a previous post there being no time limit as far as I am aware regarding basic piston engine power plants & I would imagine that this is because the manufacturers would have no way of controlling any said limits once it left the factory anyway. I mean there would be a lot of pilots out there whom just run the engine anywhere within it's operating range inc full power all day everyday with minimal care.
These power plants would have been tested well beyond their normal operating regime to gain certification anyway.
Bit like the SWL of a crane, it would have been tested to destruction to find it's max limits them probably reduced by half.
Wmk2