The Nielson Company -- the uber-powerful organization which tracks the viewership ratings by which TV programs live (like Lost) or die (like Studio 60) -- has determined that World of Warcraft was the #1 most played PC game during the period of April - November 2007.
According to the group, residents of Azeroth and Outland averaged 17 hours of play per week -- 12 hours more than the nearest competition, players of The Sims. So when my family and friends suggest that I'm crazy for playing WoW more than 15 hours a week, I can point to this and say, "Well, if I'm crazy, then so are at least 4.5 million other people!" Yeah, kind of a lame defense, I know.
17 hours seems like a good estimate to me because it falls smack dab in the middle between your casual players (who probably play up to 10 hours a week) and your hardcore types (who probably play 30+ hours per week).