You can run a motor system at full throttle for about 10-15 seconds, but nothing more than that. Usually, before I fly, I run my motor system to full, just to check that it is working properly. It's only very quick though.

I have seen people, and have read of people running a motor system at full throttle (brushed and brushless) for the entire battery on the ground, because they said they need to break the motor in.

Some tests I have read about state that the break in period for a brushed motor is not needed, as it only yields about a 5% effecieny increase-time that could have been spent in the air breaking the motor in.

Oh, and the people who ran brushless systems at full throttle overheated the motor and ESC, and caused them to go out. Just to show that you do NOT need to break in brushless systems, which is where the term "Plug N Play" came from