It depends if you are freezing it in a chest type freezer that doesn't defrost or if it is in the freezer part of a fridge.
In a chest freezer kept nice and cold (0F) and correctly vacuum sealed (no freezer burn), I find no degradation with fish, whole trout/kokanee or salmon fillets, that have been frozen for a year.
In a fridge freezer, because it periodically defrosts and usually isn't as cold, I would probably say 3 to 6 months...
cheers, roger