The Embers alternate narrative starts with 'The Cave of
Two Lovers' E2, and as of Chapter Fifty One (565k words) is covering cognates to
'Sokka's Master' F4.  There has been strong foreshadowing for events covered in 'The Day
of Black Sun' F10 and F11, so the story is unlikely to end before then.  Given the scale
of the problems, and the implied desired methods and end state,  I am expecting the
narrative to last to 'Sozin's Comet' (F18-21) or later.  I expect Embers to end up
huge.

Embers can be considered a cross over with the nastier grade of fairy tale.  The setting is one suitable for telling stories of supernatural horror in.
It is known in canon that there have been many Avatars, and that while there may have
been a time with bending and no avatars, there was certainly an earlier time before
humans had conventional bending.
In Embers, there have been hundreds of avatars, each with the potential to live
half a millenium or more.  This means that a conservative estimate for the history of
the Avatars is greater than our own recorded history.  I tend to go with an estimate of
a hundred thousand years, and consider it middle of the road.  
In Embers, Humans settled the Avatar's world from elsewhere.  This time period may have
been the one when the offense which motivates certain spirits occured.  (Certain spirits
have been offended by humans, and think that wiping humans out will resolve this.)  This
may also be when the Great Spirits were named by humans and had their natures redefined
by the process.  Some time after coming to the world, the population started to seperate
to align with the animals they learned bending from.  Then a plague occured, which seems
odd, as these humans seem to have had at least comtempory technology, as well as what
was either very advanced healing technology or magic.  Then the wars really started.
That the Avatar system in Embers was able to keep any of the four populations from
wiping another for a hundred thousand years or so says a lot about its power to me.  
That the wiping out humanity plan has come along so far in so short a time against such
speaks of the determination of its backers.  Frankly, as we as readers are just now
getting into the meat of the whole mess of plans and counterplans, I expect it to take a
good long while for the characters.

We learn more about Humans being alien to the Avatar's world.  We learn about
the Volcano of Doom.  We meet Ozai's surviving Grandmother.  Ozai seems to think that
Azula may be too stable and psychologically normal to serve as heir.  We finally meet
Piandao.  And of course we also find out about the couple whom I now more strongly think
are Aang's biological parents.