How Long is Too Long?

One of these big questions I've had over the last year or so is how long is too long for a series to run. As a fan of various series over the years, I feel in some cases some should have ended sooner where as others should have gone on longer. I've give two examples: Gravity Falls and MLP: Friendship is Magic.

Take MLP:FiM. Right now we are finishing up season 6, moving into season 7  with a movie on the way. For me, I feel that the series is beginning to run out of ideas (I admit I really watch most just once and never re-watch most of the last few series due to more of them being less interesting compared to earlier episodes), which can lead to unnecessary changes to story and characters, ones that may not go to well with some people. Hell, I feel the movie idea is a few years too late as it won't grab as many people now the height of the fandom has already peaked and is now cooling down.

Gravity Fall on the other hand only lasted two seasons before wrapping up with a nice conclusion, yet many felt it could have gone on longer and had more stories to tell. Here was a show that was reaching it's peak, but cut itself too short for many fans. That's not to say anything bad about it as unlike many shows they did give it a great ending that finalized the series, but did leave it open for continuation down the road.

Don't get me wrong, both a great shows and I do enjoy them, yet one must wonder how does one tell when a series reach the point when it just has to end? I gave two cartoon examples above, but you could literally apply it to any media series (television show, comic, game etc). In my experience, somethings have gone on for so long that they get different writers in who decide a story or character(s) need to go in a new direction, changing completely without proper explanation. A character who seemed deep and who's personality was one thing, suddenly changes to a polar opposite out of nowhere and any explanation never justifies the change. This is one of my biggest fears when it comes to something I begin to follow, a steady build up and path, only for something to undo it all without the need for it. I guess it's something fans go through, but when milking a series for all it's worth, is it really worth it if it ends up destroying it and alienating a fan base?

I don't know, but it does open itself up for an interesting discussion.