Conversation so far:
What did Phil Hartman do in Phil Hartman's early career?
Working alone as a graphic artist, Hartman frequently amused himself with "flights of voice fantasies".
What did he do as a graphic artist?
CANNOTANSWER.
What were the voices he did to amuse himself?
Hartman recorded a number of voice-over roles.
These included appearances on The Smurfs, Challenge of the GoBots.
What other shows was he on?
Hartman co-wrote the script of the 1985 feature film Pee-wee's Big Adventure and had a cameo role.
Did he write anything else?
Pee-wee Herman and developed The Pee-wee Herman Show.

Question: How long did he work on the show?
Self-contained rewrite: How long did Phil Hartman work on the Pee-wee Herman show?