Place the snapper in the lightly oiled roasting pan. Bake in the oven for 10 to 15 minutes per inch of thickness at the thickest part of the fish.The fish is done when the flesh is fully opaque and flakes easily with a fork, or the internal temperature reaches 145 F.lace the snapper in the lightly oiled roasting pan. Bake in the oven for 10 to 15 minutes per inch of thickness at the thickest part of the fish.