Conversation so far:
Who does Kermit the Frog perform with?
Jim Henson originated the character in 1955 on his local television series, Sam and Friends.
What was Kermit the Frog's performance about?
Jim Henson's Kermit the Frog was coming out of his own personality—was a wry intelligence, a little bit of a naughtiness, but Kermit always loved everyone around.
Are there any other interesting aspects about this article?
Following Jim Henson's death, veteran Muppet performer Steve Whitmire was named Kermit the Frog's new performer.
Why did Kermit the Frog need another performer?
Steve Whitmire seemed to imply in a blog post that Jim Henson had asked him to assume the role before he died.
When did Jim Henson die?
1990
Which appearance had Jim Henson had before his death?
Jim Henson's last known performance as Kermit the Frog was for an appearance on The Arsenio Hall Show
What else did Jim Henson promote about the show?
to promote The Muppets at Walt Disney World

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