Let me give a bit of my experience in ground (bench) test of big fans:
We ran a performance test schedule as follows - 5 min. MCT for warm up - 5 min. at highest TO rating - then 5 min. each at a series of lesser TO ratings, then a few points 5 min. each at lower settings. All this without any cooldown settings. It was not unusual to accumulate 30 min. above MCT with no interruption. It short, we
completely ignored FAA/JAR time limits.
There was never any question of mechanical problems, although we could measure some creep of EGT/TIT and fuel flow deterioration. This would be expensive for an operator, but NOT hazardous.
Not a legal answer, to be sure, but it should put to rest any fear of exceeding a time limit.
