Some questions on break in periods....

I realize that some equipment requires a break in period before it sounds the way the manufacturer wanted the unit to sounds, so what constitutes as "break in?" Is it actually usage of the unit in question or does merely leaving it powered on through the night count as part of the break in? If the unit sounds bad out of the box, how many hours are required for break in before the listener decides the unit will never please them? One last thing; should a listener break in one piece of equipment at a time, break it in, before testing another piece of equipment? It seems to me that breaking in one unit at a time would help a person zero in on which item in the audio chain is changing the dynamics of the system.