Hi, well I am unable to help you in regard to how long this last, but wanted to share my story, cause we are kind of in the same boat.
My Emmalee started semi brumation in October as well, she was 9 months old. She was a lot slower, her metabolism was slower, and she reduced her eating. This went through to November where she only ate one super a week. By December she ate a super maybe every two weeks, on the 19th she ate her last super. She went down for full brumation December 23rd, and woke January 10th.
Since then she has still remained in a semi brumation state. She went from 358 grams to 337 grams. She is still not interested in eating, and since she woke I can count the times on my hand that she ate a bite of veg. Just a bite, nothing more. She is not interested in any feeders, whether it be crickets, super or waxies. I have bought them all to just die.
I recently started to assist feed a slurry to help get her weight up, but even at that I am lucky if she eats 3 mls per day!!! Brumation is not a good, or fun time for the owner, this I have learned. I have also learned that most brumations last 2-3 months, but Emmalee is having a harder time getting back to normal.
So I guess there is no hard and fast rule when it comes to brumation, and I have been told that some of them take a long time to get back, so maybe we just have those kind of dragons.
Only advice I can give, is to make sure the fecal checks out ok, make sure this isn't parasitic. I know Emmalee was good before all this. And from what I have learned during Emmalee's process is that your dragon is normal, they can go in and out of brumation several times.
I am hopeful that with daylight savings it may trigger a natural Spring response. I will wish the same for your Zig, good luck!