To be truthful, it takes less than a month to mount any animal start to finish, if the taxidermist has only your animal and does all of their tanning in house. Why does it take 18mos then ? If the taxidermist uses a commercial tannery, it may take a year to get your tanned hide back in their shop. When they do get it back, they probably received 50 or more hides on the same day, and the real work begins. If you are at the top of the list you may see you mount completed within the next two weeks, if you are towards the bottom it may 3 more months.
For fish and birds the timeline is much shorter. most taxidermists can set a bird in 1 day from fresh to mounted. Remember though they have to structure their work efficiently and with respect to the timeline of intake. This means they may work on fish for 3 months, then work on shoulder mounts for 3 months, then finally work on birds.
To sum it up, if your taxidermist says it will take one year, expect your mount to be done 1 year from the the day you dropped it off.
You may be tempted to message or call your taxidermist and ask how it is coming along? Is this ok? Certainly any taxidermist should expect some sort of contact from a client after a few months. Maybe its only been 6 months and you are simply anxious and excited. Maybe its past your expected completion date and you simply want to make sure the taxidermist is still alive. In any case you should expect a generic, polite, response. They may not know exactly when they will get to your animal, and don't want to give you false hopes of early completion. If its past your Completion date they ought to give you some explanation why it is not done yet, or a general idea of how much longer it will be. If its past your completion date its likely the taxidermist has already invested time and money into the initial process of your mount. its likely been skinned, fleshed, tanned, materials ordered. they have likely invested more time and money than you deposit alone covered. In addition, every mount presents different challenges, whether is a broken fin, missing feathers, or damages to a hide. These factors as well as drying time can create much variation in the amount of time it takes. Patience here is the key to receiving quality work.
In short, if you are curious, feel free to contact your taxidermist, but do not expect a conclusive response. When it is done they will contact you, up until that day they can not provide a specific answer, bur should be kind and informative.
Rightfully so, you paid the deposit, all of your buddies have their stuff back, why not you? There are a few ways to handle this situation. You could pester them every couple weeks. You could tell the taxidermist you are coming to get your mount and your deposit back. You could continue to wait indefinitely.
Do you have the money to make the final payment ? if you pester your taxidermist he may expedite your mount and if you aren't ready to pay you will look like a fool when you tell him you can't pick it up yet.
If you decide you've had enough, and you are going to get you animal and you money back, you may be making it worse for yourself. If you've already waited 18mos, you will be waiting that same time out with another taxidermist, and you will be investing more time and risking the quality and mountability of your animal as it is transferred from freezer to freezer. If your taxidermist does good quality work, your best bet is to make them aware of your disappointment and continue to be patient.