The third season left things reasonably, but it's not at a final conclusion at all. They have an upcoming fourth season. They'll probably go about five seasons. Each season is eight 1-hour episodes. Five seasons would also be about equal to the comics in terms of length, although the show has (in my opinion very favorably) deviated from the comics in a major way.

I think five seasons is the right number for this; I don't like shows that go too long unnecessarily.