14th century, Les Enluminures
"After leaving the bath, let her adorn her hair, and first of all let her wash it with a cleanser such as this. Take ashes of burnt vine, the chaff of barley nodes, and licorice wood (so that it may the more brightly shine), and sowbread... with this cleanser let the woman wash her head. After the washing, let her leave it to dry by itself, and her hair will be golden and shimmering... If the woman wishes to have long and black hair, take a green lizard and, having removed its head and tail, cook it in common oil. Anoint the head with this oil. It makes the hair long and black."
The Trotula (12th century)
How frequently you should do this is not explained, but if you are going the lizard-conditioner route, my recommendation is: use sparingly.