It all depends. If you were the first one in, you could probably get it in a month, if the forms and stuff came when they should. If he has 20 ro 30 in front of you, then you are looking at the 6-9 months.
Most taxidermists do not do a mount one at the time from start to finish. He will cape, flesh and do things like that as they come in. At the same time they will be getting ducks, bobcats, and other stuff. Once it gets to him, he has to order the forms and all that.
Then he is also doing antler plaque mounts, Europeans, hogs or what ever.
If one tells you he can get back much quicker, he probably isn't very good, because he doesn't have much business. I have had 2 done by a guy who only does it on the side when he is not rice farming. He does very good work and got them back pretty quick, but he only had a few to do for freinds and their freinds.