You should wash and moisturize your hair as often as needed to keep your hair clean, soft, and manageable. Washing removes buildup on the scalp, and dust or dirt that may cling to the hair from the atmosphere. Moisturizing with a good conditioner helps to soften the hair, aids in the detangling process, and seals in moisture. Both are essential for healthy manageable natural black hair.
How often you wash your hair will vary depending on your hairstyle and lifestyle. If you are very active (you participate in sports, or do some kind of physical outdoor labor) you may want to wash your hair daily. Of course this means that you should wear your hair in a style that is compatible with washing your hair this much. If you wear your hair braided, twisted, or thermally pressed, you may want to wash and moisturize (condition) your hair weekly or biweekly. You should wash and condition your hair at least twice a month.