Cooking time depends entirely on whether you're cooking pan-dressed whole fish or cooking fish filets, and whether you're deep-frying or pan frying.
As Uncle Bob says, with deep-fried filets, they're done when they float - probably 3 or 4 minutes. I prefer to pan fry whole fish, although you can deep-fry them if you want. Just reduce the temperature a little, to maybe 360 instead of 375, and cook them a little longer. They may not float when done, so you have to judge doneness by trial and error until you gain experience. If you have the cooking temperature right, they're done when they're browned.
Pan frying works well for both filets and whole fish and is my preferred method, unless I'm cooking for large groups. You need about 1 to 1 1/2 inches of oil in a heavy skillet (cast iron preferred). Preheat on medium-high heat (360-375 degrees), and cook for 3 to 5 minutes on the first side. turn and cook another 3 to 4 minutes on the second side. This is an art, not a science. Thicker filets or whole fish should cook longer, at lower temperatures, and thin filets for less time. After a couple of efforts you'll have the process down pat.
To me, corn flour and corn meal are interchangeable for fish coating. Use which ever you want and mix it 50/50 with white flour. Personally, I use corn flour as does Uncle Bob, but it's no big deal. Add salt and pepper and any other seasonings you like. Paprika, garlic powder and cayenne are common. Lemon pepper, Old Bay and Zatarain's or another Cajun seasoning blend are common.
I like to soak all my fish and seafood in buttermilk for 10 or 15 minutes before coating. However, don't soak in buttermilk for more time than this because the acid in the buttermilk will make the fish too soft.
Whatever else you do, DON'T OVERCOOK! That's the biggest problem home cooks have with fish and seafood. It cooks much faster than you probably think and is an abomination when overcooked.
"Iím going to break one of the rules of the trade here. Iím going to tell you some of the secrets of improvisation. Just remember ó itís always a good idea to follow the directions exactly the first time you try a recipe. But from then on, youíre on your own." - James Beard