I also had the 12/7 nerve graft preformed. Mine was done 4 days after my AN surgery while I was still in the hospital. I was told that it is best to have the graft done ASAP while nerve endings were still "alive" and viable and, as Caryawilson says, the muscles don't atrophy making movement and success of graft more likely.
I'm also not sure how long would be too long to wait. I would think 18 mos would be a long time based on what I was told. But I'm not a doctor,,,
Yes, you can also have a muscle graft. One I was told about is called a Gracilis Free Flap surgery. Basically, ( from what I remember being told) it is not a nerve graft but a muscle ( usually from the thigh I believe) graft. A muscle is transplanted to cheek area and attached to a muscle and you can get a better smile with it. Now this may only be possible with the nerve graft, I'm not sure. The doctor I was told about who is expert at it is Dr Tessa Hadcock at Harvard or Mass. Ear and Eye clinic,, you can "Google" her.
There is also the T-3 surgery that uses a tendon, I believe, from temple area. Not sure about specifics.
Good luck. Maybe your neurosurgeon can recommend something since he told you to wait,,,
Jane