Like Todd said, its been seven months since Skyrim's release and we're finally getting the first DLC in Dawnguard. Now, don't get me wrong, I'm very excited about this DLC but it does not seem that its going to be the same size as something like Shivering Isles, which I would consider a true expansion pack.
What I'm getting at is if it took seven months just to make Dawnguard, we'll probably be waiting a bit for the larger DLC(s) that Todd has eluded to. Hopefully there have been different teams working on different DLCs and there won't be too large of a gap between the release of Dawnguard and the next DLC (possibly Hearthfire), which hopefully will provide a new landmass to explore somewhere.