As a mother of 3 autistic children, head banging can be a "normal phase they grow out of" or it could be a something else. If you have no other concerns about development, then she will probably grow out of it as long as she does not get attention or her way as a result of the head banging. If she is having sensory issues or communication frustrations, this could continue.
I do want to correct something that someone said earlier that "she won't hurt herself". That may be true of typical developing children, but kids with autism will bang their head until it hurts (they will also actually starve themselves as well). This is where you need to look to see if your child is developing on a typical level, if so then wait for her to grow out of it. If this is only one of a few concerns, then talk with your pediatrician and check out more developmental information.
I am not trying to say your child has autism, I am just explaining the difference between "she will grow out of it" and what could possibly be a red flag for something more serious.