people often move to places with a language that is needed to exist in that place and then manage to learn that language. I would say becoming proficient in a language with small effort and no especial facility in languages should take no more than 2 years. With a good deal of intensive effort, focus and natural 3-6 months.

this also depends a lot on the language, many of the Romance languages are relatively easy to learn, many of the Nordic ones seem quite difficult.

Difficulty of course may depend on what you are moving from to what you will be using.