The amount of time it takes to learn an instrument depends on the instrument, the student's age, and the amount of time they practice. In general, it takes several months to a year to learn the basics of most instruments including how to read music and play basic scales and chords. With regular practice, students can expect to play simple songs within a few months.
Piano, guitar, and percussion instruments are some of the easier instruments to learn. Strings and horn instruments take a bit longer to learn. Becoming proficient at any instrument takes several years of lessons and regular practice.