Keep in mind, it could take 20 minutes of cooking over medium-low heat before the frittata is ready to be flipped. When it's just about time to flip it, you'll be able to smell the frittata getting browned on the first side. Once you get that aroma, look at the edges to see if they're set. If so, gently lift up an edge with a spatula to check the underside. When the bottom of the frittata is a deep, golden brown, it's time to flip the frittata.