I'm afraid I'm not enough of a pot-stirrer to write the type of post that would generate dozens of angry comments with shouts of "CASUAL!!!" on the one hand, followed by "NO LIFER!!!" on the other.
I'm afraid that how many hours one defines as 'too much Wow' boils down to the old adage about driving: People who drive slower than you are idiots, and people who drive faster are maniacs, and anyone driving the same speed is 'normal.'
What is 'normal' for playing Wow? What is 'too much' Wow-playing?
There are a few clear dividing lines, actually.
Reports of seizures or even death from playing 'too much Wow' are sprinkled through cyberspace. A few examples are this 2005 report of two Wow-related deaths, a boy who reportedly had seizures after extended play, and the many many forum posts about "how Wow ruined my life."
One can, objectively, play too much. While the question of whether or not Wow can can actually be addicting is open to debate, there is no question that excessive gaming can be unhealthy.
In the American Psychiatric Association's DSM-IV (Diagnostic and Statistical Manual of Mental Disorders-IV), a mental disorder is conceptualized as a "clinically significant behavioral or psychological syndrome or pattern that occurs in an individual and that is associated with present distress (e.g., a painful symptom) or disability (i.e., impairment in one or more important areas of functioning) or with a significantly increased risk of suffering death, pain, disability, or an important loss of freedom)."
It seems rather obvious to me that people's gaming can lead, if nothing else, to neglect of social or occupational commitments.
So yes, one can play too much.
Of course, few would suggest that merely playing the game is unhealthy, and millions can attest to the enjoyment derived from a few hours of participating in the world's most popular MMORPG. In fact, a fairly large body of evidence exists to support the benefits of playing video games (including this report regarding our favorite game).
Ah, so Wow isn't inherently evil?
Back to square one.
How much is too much?
It boils down to an individualized cost-benefit analysis.
Playing Wow has 'benefits' - fun/enjoyment of achievements, the rush of killing a raid boss, the pleasure in winning a battleground, social interaction with guildies, equipping that shiny new piece of gear, and so on.
On the other hand, besides the obvious medical problems possible from excessive play, dedicated Wow play can disrupt one's personal social life, and interfere with work or school (I know many a friend or guildie that has neglected/lost a relationship or left a degree uncompleted due to Wow).
During my first 18 months of playing Wow I was living on a small island in the middle of the Atlantic. I started playing when a friend came to visit and introduced me to the game. I am a night owl by nature anyway, and in order to play with my friends, I had to stay up late - I often played from 11pm to 3am my time. I also don't need much sleep, so 4 hours a night was fine with me. Of course, as I reached max level on my rogue, I became aquainted with the endgame that is raiding, and fell in love. As my skill and gear progressed, I eventually applied for and was accepted into a top-50 US guild. This required me to raid 6-7 nights a week, from 1:30am - 7:00am when I had to head for work. I slept for a few hours after putting my kids to bed, then raided all night. I also played quite a bit on weekends. While my play schedule did not interfere excessively (in my opinion, at least) with my relationships, or even my social life, it certainly took its toll and I have to admit my work suffered. Fatigue ground into me and eventually I realized I couldn't sustain the time requirements and dropped back to a more casual schedule for the remainder of my months on the island.
After my return to the West Coast, I was able to play more with my friends at 'reasonable' hours. They were a talented, fun bunch, and established the idea of the 'hard core casual' guild that we see more of today - the concept was two raid nights a week, but serious focus and drive during those times. That should be no problem! Well, the problem was, raids started at 5pm for me. I would come home from work, immediately log in, and start playing. My family had to fend for themselves. And it was difficult on off nights to decline evening ten-mans or playing with my friends who were on at that same time. I progressively played more and more, during the evening and weekend hours, eventually becoming co-GM of the guild, and helping lead our 'casual' guild to a #2 server ranking despite our limited raid times. I was king of the world! I was having my cake, and eating it too! It wasn't until my 'cake' (in the form of my then 8-year-old) asked one more time, 'why don't you play with me any more?' and my kind, gentle, supportive spouse said 'enough,' that I clued in to the costs my Wow play was incurring. Too much. I quit Wow.
The next few months were boring. I played soccer. I went to work. I played with my family. I read a ton of books. I grilled on the backyard BBQ. Had the neighbors over. WAIT!!! WHAT??? That doesn't sound boring! In point of fact, all of the above were wonderful, and I was very glad to be doing them again. But I still hung out in my old guild's forums, read MMO-Champion, and my night owl ways left me watching way too many informercials. I didn't have to play, but I still wanted to. So, with the spouse's blessing, I embarked on an attempt to better balance Wow and non-Wow time. Limiting my time to after the children are in bed has made a huge difference. Still, what to do on non-raid nights? Run 10-man content, do daily quests, play the Auction House, BGs, arena; the possibilities are endless.
Recently, I found myself in a raid every night. And found that I was missing the extra time with my spouse, and doing other activities like reading and catching a few shows (like Chuck - wonderful!). I was still, I have to admit, also slacking on my contribution to the onerous household necessities, like (ho-hum) dishes and vacuuming.
Would I like for both my shaman and my DK to do an ICC 10-man each week? Yes. Would I like to kill Arthas? Hell yes. Two groups in my guild have killed him - they work on ICC10 two nights a week, while my Friday group has gotten to him with no time left to work on a kill. Would I like to BG, arena, or maybe do an achievement here and there, or level an alt? SURE! But, for me, the answer has become more and more clear. There are benefits to playing 7 nights a week, and to doing those things listed above - they're fun! But there are costs as well, and the costs are too high.
So, while I'm not in a top-100 guild, and I still haven't killed Arthas yet, and there is much yet left undone in the World of Warcraft, I'm perhaps at my happiest I've ever been in my Wow-journey. I still raid competitively. I still min-max my main toon and my raiding performance. I still play a lot by many standards. Yet I also have time to do many of the things outside of Wow that I enjoy. I'm not perfect, but I'm happy with being a work in progress, and in getting better at finding a balance.
So, what does your cost-benefit analysis look like? Do you play Wow too much, or too little? What things might you be neglecting for Wow, and should you devote more time to those other things?