How long do I hold them down for? Is something suppose to happen with the machine?
The computer will eventually startup, once it starts up you should be fine.
That sounds misleading, as if you hold them until the machine starts up.
The correct procedure according to Apple is you press them down together once with the power button and let go. You then Wait 5-10 seconds and then press the power button again as normal to start up your computer.
I was going with my experience with other machines, coupled with the fact that the battery was built it. I just assumed that you would hold it down, the PMU would reset/cycle, then the computer would activate.
TEG
My MBA did not restart, no display, no keyboard light and pressing the power button for long did not help either. In the end saw the link above and followed these three steps mentioned in the link and my MBA came to life and is working now.
It is a PMU reset, with the computer off, hold down Command+Option+Power+Shift.
TEG