I hit the halfway point of G project (reading wise), two nights ago. So far the voice hasn't drifted nearly as much as I first feared. On the other hand, I realized that there are a number of parts that need to be expanded. That's ok though, it's better than voice drift.
Currently, G project is at 56,000 words. I have three more main cruxes to get through, and then the book is done. My guess is that to end the book will be another 10,000 words. After that, I'd say that I will probably add another 5,000-10,000 words expanding the sections that need help. All in all, it's about a month or two worth of work.
It's amazing to me how much smoother writing this book has gone as opposed to Continuum. I think it took me two years to fully write it, and G project is almost done in half that time. I can't imagine how fast G project would have gone had I not gotten a new job in the middle of the whole process.