Before when I applied it, it took about 5 half hour thermal cycles on full load to get my temps to drop a little. I assume this was due to the thermal grease burning in, in the past I have seen a drop of about 3ºC. Also, I measured the ambient temperature and always did the test when it was the same temperature in the room.
I don't mean a burn in as in AS5, just a few hours of CPU stress and its set.
Not trying to start a dispute, just saying that I believe that it does have a burn in time.