in 2018
EDIT- and can a mod move this out of the Lore forms
2018 is just a wild guess..
for all you know bethesda put another larger team on TES VI since Skyrim was wildly popular
If current trends continue, 2017 at the earliest.
I think that with the success of Skyrim, we might see it come out faster than prior games. Not much faster, but faster. So I'd say 2015-2016 is a reasonable guess.
And ya, this is not at all a lore question.