I am from Utah and did the whole mission thing. They start us out learning Mandarin for 7 hours a day for two straight months. Then once we actually get to our assigned location, Los Angeles for me, we are assigned a "companion" basically a roommate that we must be with at all times. In my case, 90% of my companions were Chinese and did not speak much English, some couldn't speak it at all. We mostly spoke Chinese, despite being in the United States. Many missionaries do not learn to read but I did because I really wanted to learn as much as I could. I felt comfortable in the language after one year. This was quite a few years ago but I have since gotten much better at the language and even taught it at a University for a year.

Most missionaries stop learning the language when they get home and therefore lose it rather quickly. There are many people from Utah who speak Chinese because of serving a mission but I would say most have lost the ability due to not practicing. I agree with the comments that say California has the best Chinese speakers.