When the bottoms of the pancakes are brown and bubbles start to appear on the top surfaces of the pancakes (2 - 3 minutes), turn over. (mymoderncookery.com)
Pancakes are ready to flip when bubbles pop on surface of pancake. (lettucebehealthy.net)
If you cook with oil or butter for your pancakes, you're making a hyrdocarbon based oil film across the entire first surface of the pancake, effectively sealing the voids created at the evaporation of liquid into the batter matrix. (sbnation.com)