Conversation so far:
When was the golden age of comic books?
the character's first appearance is usually given as Adventure Comics No.
40 (cover-dated July 1939).
Where else did he appear?
he also appeared in DC Comics' 1939 New York World's Fair Comics omnibus.
Who appeared with him?
CANNOTANSWER.
What was special about Sandy the Golden Boy?
nephew of Dian Belmont.
Who is Dian Belmont?
CANNOTANSWER.

Question: How long were they together?
Self-contained rewrite: How long were Sandman and Dian Belmont together?