Typically, you could switch your chihuahua over from puppy food to adult food from anywhere between 1 year to 3 years, but it depends on your dog's personal growth, health factors, etc. If your dog has hypoglycemia, anemia, or other factors, please ask your vet! Some chi's have successfully been kept on puppy food their entire life with just the quantity changed!
Some have been fed all-life stages food to reduce confusion, but this might not be for your dog, since all-life stages foods typically (but not always) have high caloric and protein amounts.
My personal feelings are: if your dog is a healthy weight, has no health issues, and is developing normally, about 1.5-2 years is a good time to switch. While the body of the chi may be done growing, brain development still happens for a couple years, that is my reason for not switching, because puppy food promotes healthy brain AND body development. However each chi should be dealt with on a case by case basis.