I probably spend 12 hours a day on one computer or another. But then, I'm an anomaly.
At any rate, there's an absolutely fascinating interactive graphic at the NYT, that shows how, in composite and by various demographic breakdowns, Americans spend their time. (Click to see in detail; this is not the interactive version, just a screen capture.)
I just spent fifteen minutes- twice the typical computer use each day- exploring it. And another ten minutes or so putting this post together.