Dog pregnancies are usually around 63 days. Chis probably tend to go a little earlier. I don't know how early they can survive, but probably not super early. They to a ton of development at the end of the pregnancy. Survival rate is low if they are a week premature or more, from what I have read, but some of that depends on how much work the breeder puts into them, as they would probably require weeks of tube feeding.
First time Chihuahua owner with many years of dog experience!