It truly depends on the type and quality of hair products you’re using, but as a general guideline, you should shampoo wash your type 3C hair once a week and co-wash mid-week if needed.
Women with type 3C hair can probably testify to how fast their hair dries out. It's for this specific reason that you should be sure to use a premium-quality shampoo and a deeply moisturizing conditioner consistently, every week.