Do we even know for certain that we'll see a grey box/white book for 5E?
The other thing to consider is that WotC has proven that the turnaround time between announcement and publication can be relatively short. The PHB was announced back in January, and was in our hands seven months later. It could be that we see something similar with the FR - an announcement in Jan-Feb, and a Gen Con release.
As an aside, given a sparser schedule I think we can expect to see the "release of the year" around GenCon. Next year that very well could be the Realms book.