Ed
   Let me give you different perspective.  I've been studying ON for something measurable in weeks.  However, it is far from my first foreign language.  (French, Italian and Japanese).  You will not learn ON the same as you would learn Swedish or Swahili.  You will not become "fluent" in it because you will not use it as a communicative system with another human being.  Rather, you will most likely sit in the middle of a pile of books and laptops, which you will use to decode or decipher the text.  As time goes on, you will rely on the reference materials less and less, as you remember more and more.  But the ON language in your brain will consist mostly of this.  It will not be connected to memories of personal, physical experiences you have had.  It will lack the positive reinforcement of successfully communicating with another human being.  (If you're familiar with Piagetian Constructivist learning theory, this will make more sense.)  If you have studied other second languages, think back:  some of the language you know best is tied to the moment you learned it.  Somebody on a bus, a key line in a movie, etc.  You're not likely to get that studying ON texts.  Doesn't mean your experience won't be fun, informative, successful, satisfying--but it will be different.
 
One man's opinion--your mileage may vary!
 
   Alec MacLean