Definitely every day, sometimes more. If I skip a day, it looks flat and greasy and just nasty...some people I encounter could stand to wash their hair more often as well.
Having long hair, it has a tendency to pick up odors around me, so even if I showered in the morning, if it spend the day at the farm or in the gross anatomy lab, I need to wash it again at the end of the day, so sometimes wash hair twice a day. I try not to do that, because it starts to dry it out. If I need to, I will use a good conditioner (I don't always use conditioner...depends on the shampoo I use, some dry out my hair more than others).
I really have never noticed any damage from washing, even washing often. It's the blow-drying that damages hair, so I usually let my hair air dry.