It's hard to say to be honest Getting the ROMs running was by far the most difficult bit, but it's probably not the most time consuming! One of the tasks is to write a JIT recompiler to reduce CPU usage to minimal levels (the current interpreter consumes 70-80% of a CPU core). This is 2 months work at least. The rendering engine and tools are probably another 6 months work.
The current ball-park plan:
June 2013 - Layout designers who are interested start helping with initial beta testing / feedback
October 2013 - Wider beta testing from normal users
December 2013 - First public release, project open sourced.
Beyond - Possibly the addition of other techs that Guitar isn't planning to incorporate into Amber (MPU5/Scorpion 4)
What I want to do is release something that looks great and runs really well. I'd much rather take my time to get it right than end up rushing it and release something that's half-baked. For this reason the dates might slip
Personally I would love to see the BWB video machines emulated. I know it has been emulated in mfme4 but it's not likey to get released. Mame is a very slow burner, I don't know how far they are on with this tech.
J