That is kind of like asking how long humans live for - we have averages, but that is not a guarantee of their lifespan. I've had to explain that to folks whose pet is 8 years old and has cancer and are shocked - they seem to think that they "all live to 13", or tell me their aunt's dog lived to 18.
Average for chihuahuas and other toy breeds seems to be 13 to 15, I've seen some older than that (16, 17). It does depend on many factors, especially their overall health. I've seen 10 year old little dogs that were "older" than some 15 year old dogs, just because of them being in poor health (dental disease, heart disease, obesity, etc.)