Conversation so far:
what was Norman Rockwell's most famous painting?
His Sharp Harmony appeared on the cover of the issue dated September 26, 1936; it depicts a barber and three clients, enjoying an a cappella song.
what was the theme of most of his paintings?
cartoonist.

Question: for how many years was he actively painting?
Self-contained rewrite: for how many years was Norman Rockwell actively painting?