It really depends on the day as to how much I'm watching. I probably average around 5 hours/day during the season. Maybe 2 per day off season.
Must have cable stations for me:
Discovery (Mythbusters)
History / History International (Modern Marvels)
SciFi (Battlestar Galactica - one of the best written shows on TV!)
Science (Survivorman)
Travel (Anthony Bourdain - No Reservations)
Comedy Central (South Park, Reno 911, any stand-up)
Military (all the Wings shows, anything WWII related, anything Cold War related)
My TV watching has taken a signifigant hit since I signed up for XBox Live. I'm down to 1-2 hours now.