It depends on the other ingredients. If you use buttermilk or seltzer (which you should), then don't let it rest more than 5-10 minutes.

I think people commenting need to qualify what they consider a pancake in their posts. Baking powder is definitely new to me, I'm used to { flour, eggs, milk }. With baking powder you'd get a sort of... flabby dough thing? And... pancakes for breakfast? I'm guessing its something US-specific.

And if it is a US thing, I'll just mention the pancake soup here.