As a point of reference - and I'm speaking only of mechanical integrity - I used to conduct engine runs on the "test bench" (test cell) with the following schedule:
Startup, mechanical check, then push up to MCT for 5 minute warmup.
Advance to the highest TO rating, hold for 5 minutes, record data.
Retard to the next lower TO rating, hold for 5 minutes, record data again.
Repeat, stairstepping down to each lower rating and taking data again.
The total run might take 90 minutes, with half of that
never below MCT. We never had any sign of mechanical distress in nearly ten years of this schedule, although some performance loss was to be expected.
My point is merely that the 5 / 10 minute certification limit is to keep everyone on the up-and-up, but the engine can run well beyond this "limit" without fear of failure - so long as rpm and temp etc. limits are observed.
