It is an obsession when your interest / preoccupation takes away so much of your time that other domains, like your health, family, friends, study or work are neglected.
I become so focused on my preoccupations that it even effects my dreams and it is the first thing I think about when I wake up... even before coffee 
Not sure if there is really a line, if you want to spend hours a day pursuing your interests and are able to pay your bills, go to school/work and take care of yourself then that is fine right?
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Glass is half full kinda guy, learn from things that go wrong in your life and ask for help when needed!
AQ = 43/50, EQ = 10/80, SQ = 42/80, FQ = 32/135, Eyes Test = 24/36, AspieQuiz = 101/200 - 81/200