in terms of casual listening?
honestly, maybe ~ 1-2 hour a day on average
If "listening" includes subconsciously listening to the music that is pretty much always around us, then much, much more. also it depends what you include. I wouldn't consider the practicing an instrument really as listening to music. or piecing together a digitally sequenced track. Listening would be the final result.
it takes me around 25 minutes to get to school, where I have music playing in my car. so that's 50 minutes. and sometimes i just prefer the quiet drive. every now and then i'll have music playing while i'm getting ready for school. that's around 10-15 minutes.
i'm at school 6-9 hours a day.
during my breaks when I'm doing homework alone, I'm not one of those people who can plug in their IPOD and chug away; I get too distracted to actively listen to music and do my schoolwork at the same time. Likewise, I cannot write and read while simultaneously listening to music. UNLESS, if i'm writing about or discussing the music at hand (i've been on NG since mid 2007, and i've written around 750 music reviews- that was mostly within my first two years). and i do alot of reading and writing.
Then there's eating. if i'm eating out, i usually ignore the radio or whatever music is going on in the background because it usually doesn't hold my interest. (same goes for the music that is played at my workplace.) if i'm at home, sometimes i will watch television, and actively listen to the soundtrack, but I would rather read.
video games are probably my #1 source for music, but I rarely make time for them anymore :(
it takes me significantly longer to fall asleep listening to music, then to fall asleep not listening to music.