“A long time.”
Don’t be fooled by advertisements that promise you can learn a language in 30 days (or some other brief time period).
After two years of total immersion, we began to feel a certain comfort level in our new language. However, it was only in our fifth year of total immersion that we felt we had the ability to express ourselves precisely and with a low level of distracting foreign accent.
Answer from Howard in Oklahoma, who has served with Church of the Nazarene in Italy and Haiti for 15 years.
“Language learning is lifelong!”
The short answer to your question is “it depends.” It depends on the language, the person, the time devoted to it, and the learning method applied. All languages are not equally difficult. A person’s first language can also determine which languages will be more difficult to master.
All that said, you should plan on at least a year of intensive language study to get to a decent conversational level. And, as I tell our new missionaries, “intense” does not mean three hours a week with a tutor and a textbook or a few hours on Rosetta Stone! Intense usually means eight more more hours a day, immersed in the language, approaching it from different angles.
The learning system is also important. Good methods can be learned from places like MTI. The Growing Participator Approach developed by Greg Thomson is gaining widespread usage as a good system that seems to work in various contexts.
But even once conversational, you don’t want to stop. A missionary should continue intentional language learning throughout his or her service. Move into literature, poetry, etc. The deeper you go, the more you’ll understand the culture.
Answer from Jim, who served in Kazakhstan for ten years and now is a vice president with The Mission Society.