I have one special interest that has lasted from about 10 years old until now - so a bit over three decades (gaming), but the form it takes has changed over time. Gaming when I was 10 was mostly paper, pencils, dice, maps, game pieces, and so on - and I don't mean board games like Monopoly, but war games and roleplaying games. But the flaw with these games then was that I couldn't play them alone. The benefit was they came with a lot of pieces (some games I had came with hundreds, even though you'd only use a few at any given time. I'd spend hours stacking and lining them up, and dividing them up by category...more than I actually played the games themselves).
When video games caught up with and surpassed the kind of detail you could get in these games and you could play them alone, I started shifting to them over time. I am still interested in my old wargames and roleplaying games, but I really enjoy computer games a lot more for not needing other people to engage in them. Also, each game can itself be an interest - Mass Effect and Dragon Age are my two most recent. Longer-term games have been Neverwinter Nights and Starfleet Command, both based on games I played the old-fashioned way. Other games that have no specific relation to anything else but are definitely interests are the Elder Scrolls games, Doom, Half-Life/Portal, Freespace, Diablo, Starcraft, and Warcraft. While my preference is for single-player, probably my longest-term computer game is Warcraft (starting with Warcraft II in the late 90s), and currently World of Warcraft.
... so yeah. Some interests can be long-term and somewhat stable. I also get new ones every so often - this year I've added ADHD, and autism was a kind of interest for the past three years but really became full-blown in the past two months. Thanks to those and my more general interest in mental health, I'm starting to get into neurology and psychology as well. A few years ago social justice became a big interest because of personal investment. Also what got me into gaming was my interest in science fiction and fantasy (which started as an interest in specific books by specific authors or specific television shows and moves, and I expanded only when I wanted to find more books, television shows, and movies that fit those interests).
No matter how many I have, I tend to focus on one at a time. Sometimes I would like to be able to give more attention to two things at once, but engaging in my interests in general is pretty satisfying, even if I am neglecting something I would also like to do (like now).