The fish will need to cure for several hours. This curing process eliminates some of the moisture from the inside of the fish while at the same time infusing it with salt, which will help preserve the fish.
How long will you need to cure it? At least 8 hours, even for thin fillets. I do at least a day for a thick fillet such as salmon. If I had sturgeon steaks or something even thicker, I might go two days.
Can you overdo it? You bet. Your fish is essentially being pickled and brined in this solution, so the longer you keep it submerged in the brine, the saltier it will get. Under no circumstances should you brine for more then 3 days, and even that will leave you with some seriously salty fish.