I'd be surprised if we didn't have some info by 6months out. (May). Considering that coincides with Morrowind's 10th anniversary, it's a good PR opportunity to make an announcement.
If that slips by, if we don't have a word by 8months out(July), I've either grossly overestimated Bethesda's capacity, or grossly underestimated the scope of what they want to deliver with the new content.
Anything up to, and over 10months(September) is going to be living in the shadow of Bioshock: Infinite.
Anything over a year is just too long to go without any information.
Just some Comparisons to previous Bethesda games. As I said before, Skyrim is a lot different.
Approximations:
Morrowind: Tribunal(6months), Bloodmoon(13Months)
Oblivion: Knights of the Nine(8months) Shivering Isles(12months)
Fallout3: OpAnchorage(3months) ThePitt(6months) BorkedSteel(7months) PointLookout(9months) MoShipZeta(10months)
Bolded-Expansions that add landmass
Tribunal added landmass to morrowind.