A Shih Tzu needs a bath about every two weeks. Bathing them every two weeks or so keeps the oils in the skin from building up.
Shih Tzus are very clean dogs. Bathing them any more frequently than once every two weeks is not recommended as this removes too much oil from the skin and can lead to dry, itchy irritated skin.
As with any dog breed, a quality pet shampoo is recommended for Shih Tzus. If the hair is kept long, a good dog conditioner is also recommended to remove tangles and keep the hair soft and shiny. You know it's time to give your Shih Tzu a bath when his hair begins to look dull and stringy.