Errmm... once a month, maybe? When it gets kind of stinky? Lol, DD2 hates having her hair washed so I generally don't bother unless it's getting really foul - her hair is super-thick and curly and actually looks best with a bit of natural oil to keep the curls in nice little ringlets, so I try to avoid causing her the stress of getting shampoo in her eyes! DD1 likes having her hair washed but isn't keen on sitting still while I brush it so I tend to just wet their hair and let it do its own thing