Deep Frying fish takes around 5 minutes to fully cook.
When cooking fish in a deep fryer you should bring the deep fryer oil up to temperature where it gets hot.
Then place the fish into the deep fryer with the oil hot and then let the fish cook for 3 to 5 minutes depending on how well done you want the fish.
I cook my fish in the deep fryer for 5 minutes after preheating the oil in the deep fryer.
After 5 minutes the fish is nice and crispy and ready for me to eat after it cools down.
You can tell when your fish is fully cooked because when the fish is cooked the fish will become opaque and flake.
You can check the fish to see if it's done by poking the tines of a fork into the thickest portion of the fish at a 45 degree angle and gently twist the fork and pull up some of the fish.