How many days does a female shih tzu bleed when in heat?

Female dogs go through a heat cycle, called estrus, every six months. She will have proestrus, or a bleeding cycle, before her true heat cycle during which she is very receptive to the advances of male dogs. This heat cycle lasts for several days up to three or four weeks. Some female dogs will undergo personality changes during this time, the most common changes being anxiety and irritability.