A cast iron skillet needs reseasoning when the food starts to stick, gets discolored, or you get rusty spots. At that point, scrubbing it out with mild soap is the way to go followed by going through the seasoning method. A quick and dirty way that the old seasoning is stripped out of cast iron skillets in restaurants is to burn it out by flipping the skillet upside down over the stove top burner. I've done similar by setting the skillet right side up on a camp stove outside, and getting as hot as possible.

I like to do my fried eggs in a six inch cast iron skillet, flipping them by tossing the skillet. No spatula needed.