Answer by taking a quote from the following article:

Tristano was born in Chicago on March 19, 1919. His mother, Rose Tristano (nee Malano), was also born in Chicago. His father, Michael Joseph Tristano, was born in Italy and moved to the United States as a child. Lennie was the second of four brothers.

Tristano is regarded as one of the first to teach jazz, particularly improvisation, in a structured way. He taught musicians irrespective of their instrument and structured lessons to meet the needs of each individual. Lessons were typically 15-20 minutes in length. He did not teach the reading of music or the characteristics of different styles of jazz, instead challenging students in ways that would allow them to find and express their own musical feelings, or style.  Foundational elements for a student's learning were having a concept of (principally diatonic) scales as music and a basis for harmony. One of the teaching tools often used by Tristano, including for scales, was the metronome. The student set the metronome at or near its slowest setting initially, and gradually increased its speed, allowing a sense of time to develop, along with confidence in placing each note.  Tristano encouraged his students to learn the melodies of jazz standards by singing them, then playing them, before working on playing them in all keys. He also often had his students learn to sing and play the improvised solos of some of the best-known names in jazz, including Parker and Young. Some students first sang solos from a recording slowed to half the normal speed; eventually they learned to sing and play them at normal speed. Tristano stressed that the student was not learning to imitate the artist, but should use the experience to gain insight into the musical feeling conveyed. Such activities stressed the value of ear training, and the idea of feeling being fundamental to musical expression. All of this preceded having the opportunity to improvise during lessons.

how long did he teach?