If you look at the definition of "obsessed" it's something along the lines of excessive preoccupation in your mind.
Anime and fandom is a hobby. This means that you should only be preoccupied with it during your hobby-time.
If you're in school and if you can't concentrate on what the teacher's saying because you're thinking about your Yu Gi Oh card collection, then that's "too" obsessed
If you have a date with someone you're interested in and you cancel it because you'd rather watch a new DVD you just got in the mail, that's "too" obsessed
If you forget to eat meals, forget to exercise, or forget to groom yourself because you're too busy in theO chat talking to n00blets, then that's "too" obsessed
HOWEVER...
If your work/homework/social obligations/etc. are all finished and you find your mind preoccupied with watching an anime, then that's a healthy and happy obsession.
You should always try to balance:
Hobbies/Play/Relaxation
Exercise
Social
Work
Intellectual
There are ways to do many of these at once. For example, if you're still in school, work and intellectual are both about the same thing. Also if you play sports with friends, then you're doing exercise and social simultaneously.
Just remember that even if you're a super-otaku, that doesn't mean you should neglect the other parts of your life.
