Autism for the past 10 years, I am compete and utter autism geek. I do some free lance training in addition to my university mentoring role, I should be paying them. I get to talk about my special interest for 5 hours straight and no one walks away or tells me to shut up....perfect. I have had lots of other during my 54 year on the planet.
I have been an astronomer since I was about 10 and it was my special interest for years, but now its just my hobby. I don't think having a hobby is a special interest in its own right, it's much more than than. A special interest occupies much of your waking hours, it's something you want to share with others, it's probably all you want to think or talk about. You can't choose a special interest, it chooses you, and once it does I don't think there is much you can do about it. A hobby may become a special interest, or then it might not.
When do special interests end? For me, usually when there is a deal breaker of some kind, perfection which can't be obtained, not being able to single mindedly peruse it. When a special interest canes to an end for me its often very sudden, and if it does there is no going back. It's like trying to rekindle a dead relationship.