I honestly don't know how much, I've never really gauged it. Too much. I'd say 80% of my free time. In defense of that, I watch all my TV, get all my music, all my movies, do all my banking, buy my gifts, etc. on the interweb.
Why? I don't really know. Habit? Addiction? Convenience? General disdain for most human contact (or is that social ineptitude? Meh, who cares?). All of those? None of those?