I agree with 83awsm. If MLP devolves from FiM into, well, how previous generations were, than my interest will go with it. But, even if such a thing happens (And I can't foresee it happening), the community will keep on burning for quite some time.
Friendship is Magic is a great show, with an unlikely fanbase (us). The fact it's popularity has reached the height it has, especially among the unexpected demographic (us), leads me to believe they'll try to keep the appeal of the show alive as long as they can.
The art is great, the story is great, the characters are great, the VAs are great, and I can't really foresee myself losing interest unless, like previously mentioned, it devolves. Maybe if they took it in some weird direction, like giving the mane characters the ability to fuse Transformers-style or something, I'd lose interest. But what are the odds of that?