It'll be available when it's finished. My general advice as far as gaming goes today is for people to enjoy what they have and to not get too excited over reported new games and expansions. I say this because about 90% of what's ever released is drek. If a new game or an expansion is good, you will read reports about it and then you can get excited about it.
In the meantime, if you're getting bored with the game as it is now, have you considered playing mods and custom maps or even playing it in online multiplayer with and against real people? Learning the online game and confronting its challenges should take your mind off the next expansion until it's ready to come out.
