There are three factors you need to consider the time to figure out how often you need to wash it.
Dry or Oily
If your scalp is kind of normal to dry, just wash once or twice a week. If the scalp is oily, do it more often, according to Columbia University.
Curly or Straight
Hair texture affect how quickly sebum walk from root to tip of the hair. People who have wavy or curly hair, tend to have their hair is drier than straight hair. The Curls only needs to wash once a week. While those who have straight hair shampooing is recommended twice a week or more.
Styling Hair
Another thing to consider is the styling or hair style. Alternated models and often coloring hair, can cause the hair to become brittle. "In conditions like this, you are not advised frequent shampooing" says Dermatologist Paradi Mirmirani of the kaiser permanente Medial Group. You must hold a trial to determine how often you need wash it. Try to leave the house without shampooing. See if your hair fells okay during the day. If not, it means you have to wash it tonight.