I cowash weekly and shampoo once a month or as needed- which ever comes first. I can't exclusively cowash because my hair gets weird and dull and my scalp gets just itchy and nasty. But I can go at least a month with no shampoo.
Have you tried doing brown sugar scalp scrubs? They really cut through scalp buildup well. Just add 1 tablespoon of brown sugar to the conditioner for one co-wash, then give yourself an extra good scalp massage with it.
Baby Fine 3B, low porosity, normal density and elasticity
CGing since July 2008