Who were some of the people that Michelangelo trained under? How long did it take for Michelangelo to paint the ceiling of the Sistine Chapel? Why was it so dangerous?

As a young teenager, Michelangelo was became the apprentice to Domenico Ghirlandaio.  Ghirlandaio was a famous painter in the Italian city of Florence.  As an apprentice, Michelangelo trained under Ghirlandaio and learned some of his skills.  Some of the skills that Michelangelo learned specifically in the field of sculpture were learned from Bertoldo di Giovanni.  He was a friend of the Medici family, who Michelangelo had grown close to.  
Michelangelo painted the ceiling of the Sistine Chapel in Vatican City over a period of approximately four years.  His work began in 1508 and ended in 1512.  Painting the ceiling was both dangerous and exhausting.  Michelangelo created his own scaffolding to paint the ceiling.  It was makeshift scaffolding, and was not very sturdy.  Michelangelo had to turn his head up toward the ceiling as he stood on the scaffolding to do his work.  This was exhausting as he tried to control his balance.  A fall from the scaffolding could have resulted in serious injury.
https://www.britannica.com/biography/Michelangelo