How often should I wash my hair?

Wash it when it's dirty, sweaty oily or gross. The idea that washing is bad for the hair or shampoo damages hair is a a modern myth; an urban legend and simply not based in any kind of scientific fact. Your scalp sweats, excretes the same sebum that clogs up the pores on your face, and worse. That build up of oil, sweat and dirt on the scalp is far worse for your hair than any shampoo or conditioner. That gunk on your scalp can lead to dandruff, fungal infections, bacterial build up, dermatitis and all those can make your hair fall out at the roots. The idea that washing is bad and shampoo is terrible originally came a from a series of television ads that sold expensive hair products. The ads made washing hair sound like the worst possible thing to do and many people repeated it until it became the urban legend it is today. The products from the commercials are now off the market and the subject of a class action lawsuit because they didn't clean the hair and many ended up with scalp problems and severe hair loss. So wash your hair when it needs it, use a conditioner after shampooing and keep the heat and chemicals off it and your hair will look nice everyday.

whenever you sweat or it gets dirty...

Wash it every second day. That is enough.

Everday

you have to see how your hair reacts. i have very oily, thin hair but lots of it, and i just wash it when it gets oily. so whatever works for you, because your hair is probably different from others that only wash it twice a week.