Conversation so far:
What was Norman Rockwell's most famous painting?
Among the best-known of Norman Rockwell's works are the Willie Gillis series, Rosie the Riveter, The Problem We All Live With, Saying Grace, and the Four Freedoms series.
What was the theme of most of Norman Rockwell's paintings?
Norman Rockwell's famous illustrations were of everyday life.

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