When I was a teenager I had to wash my hair once a day partly because it was that greasy and partly because it would stink like the barn after doing my morning chores. Didn't want to go to school smelling like the barn.
When I was going into the office every day, I would wash my hair every other day (so Mon, Wed, Fri).
Now that I work from home, I'm a complete slob. I wash my hair Fri and Sun. I might wash it mid-week as well if it's buggin' me or if I work out.
If I'm going into the office say one day during the week, I'll wash my hair that day. Oh yeah, and I will shower too.