Spend a good 30 mins washing and making sure that your hair is fine, I'm sure she'll be appreciative of the time you've spent on it, and it won't be charged into your time at all....
In reality, and if your question is actually genuine, then don't. Spend your time in the shower making sure your important areas are clean and smelling fresh, as that's what she'll be worrying about. Spend a couple of minutes cleaning those areas, and you're ready to go. I'm sure hair is the last thing you should be thinking about (Well, the hair on your head at least).