How long can a duck stay in the freezer for taxidermist?

If it isn't freezer burned and you took care of it, it should be. Let the taxidermist look at it and tell you. If it's a reputable one, he's not going to mount it if he can't make it look good.

Funny you should ask, my brother in law had a pair of redheads mounted that was in the freezer for 15 years and were mounted and looked like they were killed this year. As long as they are wrapped right and not freezer burned they will be fine. He literally forgot about them and just found them in the freezer. He got him a new freezer and came across them when he was swapping stuff out. The guy who did the work had to take his time thawing them and working on them. It’s an awesome story about the birds how they were taken and then being forgotten then found them in the freezer some years later and then the taxidermist has not been long doing work but he brought them back in an amazing way.