i think we will prepare are more detailed description of how we plan to progress with the content releases some time soon.
what i can share so far is that we certainly won't make you wait over a year or something for each new Tier, like you are probably used to from other places.
On the other side, as you mentioned, we cannot release more content too early and make the total progression reach its end after one short year.
We will keep an eye on the server population and the pace of progression and find the right time to release the next part of content.
2 things are important:
1. - we can't wait too long, because the next tier of content should not get imbalanced right at the start, because guilds go in there totally overgeared.
2. - we can't release more content instantly, or too shortly after the previous tier of content has been cleared, because then we would reach the end of the road too quickly.
I'm sure we will find a pleasing pace of content releases that makes everyone happy.
On retail Naxxramas got released 19 months after the US launch, i think something similar to that would be desireable. wouldn't it ?