this is exactly the kind of game that brings out addictive tendencies in people who already have them. They've dealt with it in Sweden and elsewhere in the world before, and the most recent place to step up and face the problem is Singapore. This article talks about a three-year study that reveals some interesting facts about how and why Singaporean primary and secondary school students play their MMOs. The average amount of playtime per week, according to the study, is 27 hours. That's over a day of playtime per week.
Still, while that seems high (and remember, it's just an average according to this study, which apparently isn't done until the end of the year), I like the government's take at the end of the article: regulating it, either by attacking the games or putting unenforceable rules on the Internet, just won't work. If these kids are playing too much (and generally the rule is that if your life is suffering, you're playing too much -- it's hard to believe that a kid going to school can play for 27 hours a week and still have time to do homework and sleep as they need to), then it's their and their parents' responsibility to get educated and fix it. Lots of people play MMOs, but it's up to those who do it unhealthily to get help for themselves.