How long does a long-hair Chihuahua's coat take to grow in fully?

I have a four-month-old female Chihuahua. Her father was a pure-bred long hair. Her mother was half long-hair, half standard.

How long will it take for her coat to grow in fully, under normal circumstances? It's not short, like a standard Chi, but it is not as long as her father's coat, either.

Also -- she has some thinning hair on her scalp, which I understand is normal in most Chis. But she also has some thinning hair along her spine on her back. I did take her to the vet and the vet did scrapings two weeks ago that came back negative for parasites (mange) ...

I do plan to have her spayed as she is only about two pounds now and probably won't weigh more than three or four pounds.

Thanks in advance.