Although a female Shih Tzu has her first heat at 6 months, AllShihTzu.com states that it is not safe to breed her until her second heat at least. She should only have three litters total and stop breeding once she turns 7.
AllShihTzu.com cautions that when breeding two Shih Tzus, the male should not outweigh the female by more than three pounds.
The American Kennel Club, or AKC, splits the heat cycle into four separate parts. It begins with proestrus, which lasts nine days. During proestrus, a female dog starts attracting suitors but doesn't allow them to breed with her until the second part, estrus, which lasts another nine days. She is fertile during estrus. After estrus is the diestrus period, which can last 60 to 90 days, when progesterone takes control of her reproductive tract. This occurs whether or not she is pregnant. During diestrus, false pregnancy symptoms are possible. Diestrus is followed by anestrus during which no sexual activity takes place for three to four months.
Once a male dog reaches sexual maturity, he is ready to mate any time, according to the AKC. However, the AKC does not register litters from males less than 7 months or older than 12 years.