Depends. If you trimmed off all the fat and meat and vacuum sealed it, could last for years and years. Left pieces of fat and meat on it and wrapped loose in a garbage bag, it's probably freezer burnt and useless...
If it was tanned, It will be good for years. I have them tanned first if I know it is going to be a while before I get around to using it. If not, Triple BB is right on. Have your taxidermist take a look at it for you.