Bake for 8-10 minutes, and check to see if they are firm. Mine baked for 24 minutes, but this is not my usual oven and I'm not sure I had it preheated well. Remove the cupcakes when they are firm and a toothpick in the center comes out clean.
Enjoy! Store in the refrigerator.
Notes
A note about substitutions: because each type of protein powder has a distinct flavor and texture when baked, I would not try substituting different types of protein powders in this recipe (whey isolate would be ok, not other types). See my Recipe Basics video for examples of what goes wrong when you use the wrong protein powder.