@Sivin How funny I was having a debate with my flatmates (who washes his hair twice a day) about this question. I read an article earlier - http://www.businessinsider.com/how-often-should-i-wash-my-hair-2015-3.I personally wash my hair every 2 days but I think this depends majorly on your skin type and the environment u're living in. Having lived in both Asia's humid area and cold Northern America, I find the need for shampooing varies. Your hair gets greasy when glands in the skin produce an natural oily substance - sebum. It is what moistens hair and keeps it from drying out, so washing your hair too often may cause imbalanced production of oil in your scalp!