Every 4-6 days. I shampoo primarily (using a shampoo bar, very gentle) my scalp mainly, and ill lather my hair if I have more buildup. In between shampoo days ill cowash but thats more to refresh & moisturize my hair (length) and its only as needed. My hair does better if I rinse and apply product to clean hair than if I add product to dry dirty hair to moisturize. Buildup is a big contributor to dryness and I also have oily scalp so cowashing only doesnt work for me.
If youre sure its not your products causing buildup another thing to consider is how youre handling your hair when you wash & manipulate it. If youre doing everything exactly the same and the only change is that youre manipulating less (by washing once a month) and seeing more growth it could be that you are breaking/weakening your hair when you wash & style it. Just a thought.