Generally, once a day. I don't sweat much, as a rule, so it used to be every other day, but my hair is quite long now and demands daily washing. I have to use moisturiser something wicked to get my skin to cooperate though...I suppose I could just cut my hair, but I haven't decided what to do with it. It's my female equivalent of a thesis beard!
All this is trumped, of course, by the need to shower ASAP if I get sweaty (usually ballet-induced).