Otherwise, through a small office stereo with the Ipod auxiliary. Headphones for something I really want to hear.
Probably 2-3 hours a day at work, on average.
Commuting, during the colder months. I will use a pair of headphones as earmuffs in the winter. However, I generally don't love listening to music while walking on the street or in a subway and really only do this to keep my ears warm when it's cold. I don't use earbuds, so pads on my ears are the only way I'll do headphones.
At home, on the main stereo.
No set time for this. Could play 3 or 4 straight while surfing the internet or reading something. Or might go an evening without playing anything and just watch movies or TV. It's extremely rare that I would listen to music while not doing *anything* else at all, but I don't think my concentration is affected by reading or surfing the internet.
I have a car but don't drive it a lot. I'll sometimes listen to music in the car, but more often than not will listen to sports talk radio. My car stereo set-up is not particularly good and listening to music in it mostly just makes me want to play the album at home.