When does a hobby become an obsession?
When you start a website about it. Make sure you pick a subject where literature and documentation are impossible to find.
Realize 99.999% of the people out there don't give a crap about your obsession.
When you buy expensive bikes that aren't even close to your size because you've never seen one before.
Reproduce some parts that will take years of selling to even cover the production costs.
Spend a dozen hours every week answering questions and correspondence related to your hobby even though 50% of people never send a thank you or a reply back.
Realize after all that you still love doing it.
Having a large collection, unto itself, is not an obsession . . . it's merely a large collection.