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. :yuck:
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.
If I miss a day washing my hair, it starts feeling yucky by midday. It hangs and touches my face and I'm aware of it all day.
I've been told though, that one of the reasons why this happens is because by washing it everyday I've hyperstimulated it to replenish the very oils I'm stripping away. The theory goes that if I shampooed it less often, it wouldn't get oily as fast as it does now.