When I was a kid, many, many years ago, we washed our hair once a week. Rumour has it, that when the babyboom ended, Johnson and Johnson's sales of their baby shampoo dropped dramatically, so they had to do something to increase those sales. Heaven forbid their quarterly earnings aren't higher than last quarter. Gosh. So they devised a little marketing ploy that exploded in our western culture, that their baby shampoo was gentle enough to use everyday! Bingo!
Personally, I have very dry hair and if I washed it everyday no matter how gentle the shampoo I would look like a sheep dog on a really bad day! lol So, I still follow what I did as a kid and wash my hair about once a week. I'm sure that if you have oily hair you will need to wash your hair more frequently. I get the most compliments on how nice my hair looks, when it is overdue for a wash, I guess when it is a little greasy it looks nice and shiny. I'm not kidding that's when people tell me how great my hair looks. You might want to take a minute to ponder that, in case you'd like to get compliments on your hair, too. :-)
My son, the scientist, informs me that our hair actually removes pollutants from the air, so the less often we wash it, the more pollutants we remove. I don't know if there was a study done on this or not, but it's nice to think I'm helping make the air we breathe a little cleaner for everyone, in a small way. lol