Do you have to bake Red Velvet cake immediately?

I am making a big red velvet cake for a party. I am using Duncan Hines Red Velvet cake & in the instructions it says after I have mixed all the ingredients to pour batter in pans & bake immediately. Do I have to bake it immediately? Is it ok to mix it & let it sit for 40 min? What will happen to the cake if I don't bake the batter immediately after I have mixed it?