There are too many variables on shelf life to answer the question without a lot more details. If you want a great resource get the book by Rytek Kutas on Meat curing and sausage recipes.
To make a safer, and very good cure for fish, try Mortens "Tender Quick" (add brown sugar, etc...). Tender Quick is nothing more than salt and sugar, with a small amount of sodium nitrate and sodium nitrite. These last two ingredients are what actually cures the fish /meat. Most people believe the salt in their brine will cure the fish, but it doesn't.