I know taxidermy, I do a little myself and I have a buddy that does it full time.
Since taxidermy is mostly a self owned/run business it all depends on the level of work ethic that your guy carries.
I know that working on your own schedule can be difficult at times with family, friends and of course all the hunting and fishing that comes along with it.
If he can't do it with an 8-12 month turnaround time, then he probably has other big agendas in his life as well.
For example, my buddy's full time taxidermy business does anywhere from 120 - 170 animals per year and still has the tenacity to turn his work over in less than 8 months and that doesn't include the waterfowl, damage repair, and fish he puts out. His work ethic though is second to none and treats his business like any ordinary 9-5 jobs with occasional overtime.
Seems during the hunting season, most taxidermist are out on some big hunting trip and can't attend to their customers' needs therefore losing time and money in the process. There can be other factors such as tanning services taking too long, damage repairs such as tears on returning capes and of course some people are just plain slow. I am sorry to hear about the turnaround time because waiting that long for any taxidermy mount will drive a man crazy.
One last thing of note, make sure you only pay the bare minimum, if the guy wants the rest of his money, he'll finish your work sooner.