I doubt it would be a never-ending series, here's how I see it.
After Katara talks about what happened, they show the title. Right now it's Book One: Water; Chapter __: ______________. Yoiu can fill in the blanks. They've only showed four chapters.
I'm willing to be that there will be four books (or four series, whatever you call it) in some order. Water comes first, and I'm betting that Fire or Air will come last (fire...or air perhaps because they want to look for an airbender or learn more air....but Ang already knows it, so why not Fire and defeat the fire nation?). Anyways, in each book/series there will be loads of chapters, and right now it's water, and they learn Water bending in this series.
It's just an idea, but I think it might end after the fourth book. And then they'll jump to reruns. Or...producing a new idea if it gets real popular. But hopefully it won't turn into Pokemon or something like that where the old eps are much better than the new ones.
Oh....Spongebob, is a threat to Society. It is the display of Idoicy and stupidity of a sponge that can talk...there's fire, under the water. Something wrong here? And a squirrel...and a...and a...pool. And a snail...snails live ABOVE water..? Sorry, Avator is the
only good thing on Nickolodeon