Conversation so far:
Who does Kermit perform with?
Jim Henson originated the character.
what was his performance about?
CANNOTANSWER.
Are there any other interesting aspects about this article?
veteran Muppet performer Steve Whitmire was named Kermit's new performer.
Why did he need another performer?
Following Henson's death.
When did Henson die?
Henson died twelve days after that appearance.
Which appearance had he had before his death?
The Arsenio Hall Show to promote The Muppets at Walt Disney World.
What else did he promote about the show?
CANNOTANSWER.
What else did Jim do shortly  before he died?
He continued to perform the character until his death in 1990.

Question: How many years had he been the performer?
Self-contained rewrite: How many years had Jim Henson been the performer?