Once per 10 days to 2 weeks. Actually, co-washing is just a light cleanse. So, how often you co-wash 4c hair really depends on your individual hair needs and preferences. If your hair is dirty and has build-up, you may want to co-wash it more frequently. On the other hand, if your hair is already quite clean and you don't use styling products, co-washing is enough. Ultimately, it's up to you to experiment and see what works best for your hair.