Salmon will stay good in the freezer just as long as you need it to, though it will not keep forever in the refrigerator. It is important to keep your salmon in the freezer if you do not plan on eating it for a while, simply because it goes bad within a matter of ways when it is left in the fridge.
You can pretty much leave your salmon in the freezer as long as you want until you are ready to use it. By freezing your salmon you will be able to avoid problems with bacteria growing on it and ingesting any potentially dangerous and harmful bacteria or toxins which could develop on it over time.