You don't smoke salmon in a brine, you cure it, one of many steps involved before the actual smoking. In my experience it is much better to dry cure for 14-18 hours, depending on the size of the filet. Any number of herbs and spices can be used, I like fresh dill, juniper berries and toasted myrtle leaves. Some acid should also be in the brine in the form of lemon or lime juice. And don't forget salpeter.
Time is much more critical here than in dry curing.