Conversation so far:
When did David Letterman move to LA?
In 1975, encouraged by his then-wife Michelle and several of his Sigma Chi fraternity brothers, David Letterman moved to Los Angeles, California.
Why did David Letterman's fraternity brothers and then-wife Michelle encourage him to move to Los Angeles?
David Letterman moved to Los Angeles, California, with hope of becoming a comedy writer.
What did David Letterman do when he got to Los Angeles?
In Los Angeles, David Letterman began performing comedy at The Comedy Store.
Was David Letterman successful from the start of his time performing at the Comedy Store?
Jimmie Walker saw him on stage; with an endorsement from George Miller, Letterman joined a group of comedians whom Walker hired to write jokes for his stand-up act.

Question: How long was he in that role?
Self-contained rewrite: How long was David Letter in a group of comedians Jimmie Walker hired to write jokes?