Usually anywhere from 5 - 8 minutes, depending on the thickness of your salmon filets. The best way to tell your salmon is done is to look at the sides of the filets. You want the fish to be an opaque pink color all the way up the sides. If only ½ of the side is a light pink, the fish needs more time to cook. You can also check the internal temperature of the cast iron salmon. When its internal temperature reads between 120 - 130° F, it’s medium-rare. When it reaches a temperature between 135 - 145°  F, it’s cooked to medium, which is where I prefer it!