When dogs go into heat (this applies to all breeds, including shih tzus) they will pass a blood-tinged discharge for 7 - 10 days, and then the discharge turns to more of a straw color and that lasts for 7 - 10 days. It is during this time period after the color change of the discharge that the females are most receptive to the male and will stand for mating. The catch here is that in some dogs, the discharge remains somewhat blood tinged, so you cannot not absolutely count on the color change. So the entire heat cycle lasts for 2 - 3 weeks and it is during the second half that dogs are most likely to breed.
Please let me know if you need additional information.