Usually once per week. I might do an additional shower if I've been excercising, or if I don't smell too good, but I won't wash my hair more than once per week.

Now before you guys slate me for having bad hygiene, you have to remember that the temperature over here is quite low all year round, so if there isn't any sweating or much bodyheat, then no need to have a shower every day. I guess it depends on your job, but IMO having a shower once or twice each day is a waste of time and water, and all those cleaning chemicals contain toxins and shouldn't be used daily... just IMHO!