The salmon needs to be brined for about 12 hours to ensure it is evenly seasoned, so if you plan on making a Japanese breakfast, it's important to start brining it the night before. Don't let it brine for more than a day though; otherwise, the proteins will begin to denature, and the salmon will get mushy. If you want to prepare this more than a day in advance, I recommend removing the salmon from the brine after a day, patting it dry with paper towels, and then wrapping the salmon tightly with plastic wrap or in a zipper bag with the air pressed out.