My estimate is sometime in late mars or april.
Ideally it should be ready, when it is ready. We are already missing too many hotly requested by players features, because Toady was hurrying up with releases. Hurried up releases are bad, because then Toady spends time adding missing features instead of fixing the endless sea of game breaking bugs. Well documented bugs, repetitiously reported and always backed with bugged file saves.
What worries me, there will be a time of entire 2 years of development for this magic system. As system, the magic system, will just add some additional functionality in both modes, as all systems always do. Still, it will take 2 years. During these 2 years we will not see any new patches for bugs, any new balancing made nor any still missing features finally added into DF both modes from what I understand.
I tried in december, january and february to play Adventure Mode. To see there, how broken from time to time is the engine, which translates the generated conceptual world and its actions into temporary world, there player acts, was to say at least quite educational. Sometimes it is not missing investment into increasing content, as missing investment into improving the game's engine, which kills modern games for players.
I hope Toady will not pause new bug fixes and old feature releases for entire 2 years and keep doing them every year like he is on right now. Even, if the new functionality of magic system we will have to wait for 2 years, which is imho ok. We can have no new functionality for one year, but bug fixes and missing features... those just have to be coming every year. It just feels so.