If Blizzard is going to make official Legacy servers, they will be announced at BlizzCon, where they make the majority of their announcements. BlizzCon 2016 is scheduled for November 4th-5th.
https://blizzcon.com/en-us/And while that is almost 5 months away, that is about the appropriate amount of time for a business of that size to work out all of the details on how it will work. They can get a jump on the implementation, so hopefully the release would not be TOO far away from the announcement - but if I were betting on a time frame, it would be Quarter 2 of 2017. (April-June 2017)
All of these opinions are for recreational use only...they are just my guess.