Well, OK, now there is a date:https://mwomercs.com/civil-war-update
By July 18th, there needs to be an NBT up and running. Ideally this would be a fully-tested NBT that is ready to go on July 18th, but it's possible that we do a beta then to give PGI time to balance patch once or twice (I don't want people to buy, say, MRM boats, only to have PGI nerf the MRMs 7 days later).
This, of course, assumes that the skill tree rework doesn't drive the few remaining players away from the game forever...
Also, as of this writing, HBS looks to have pushed Battletech back to "late summer/fall 2017":https://community.battletechgame.com/forums/threads/5953
The intent was to have NBT operational when that game comes online as well, so this lines up nicely with their new timeline (get to iron out the kinks in NBT with MWO 3062, and hopefully transition into a smooth Battletech NBT league launch).