Well, if it's something you're just interested in, then at some point you might become no longer interested in it. That's fine, it's normal, it's why people end up with an attic full of supplies for old hobbies. I've had certain hobbies and interests for 30+ years but it's not literally doing the same thing over and over again, there's plenty of variation within a topic. Always new things to learn, techniques to try, tools to use. If that variety ever stops, yep, I'll probably lose interest.
But your question seems to be more about long-term projects, which aren't something you just happen to get lucky and stay interested in. You have to actually make the decision to do the thing, set the time aside, and have discipline even on the days the magical motivation fairy didn't happen to sprinkle her dust on you.