how long does it take to "cook out" the taste of flour

If you're using the recipe from river Road Cookbook, It's delicious. The time will vary with the heat level. You'e blending the flour and butter, not cooking it a la roux. You should worry more about the blend than the color or taste. The whole dish cooks a while more so the floury taste will disappear. I hated spinach until I had this dish