When is a joke no longer funny?

When is a joke no longer funny?

An old axiom states that "a joke repeated is no longer funny."  Is this the case?  Comedy inspires repetition.  If a joke works once, it should work again, right?  There is a big difference between repeating a character and repeating a joke.  Early SNL had the right idea.  They reused Belushi's samurai, but they put him in different situations so that the joke was developed and expanded.  It wasn't the same joke each time, but a nod and a wink to the original premise.

I can't say the same for all sketches or comedy bits currently on television. I love Jay Leno, Ellen Degeneres and SNL, but are all guilty of killing a joke using it too often..Jay's "Jaywalking" is an example.  Yes, there are stupid people out there, Jay; we get it.  The same holds true when he brings in some of the same people into the studio for a quiz show.  The first time, it was funny.  The second, not so much.

Ellen loves to frighten her guests and employees by jumping out at them from behind doors, walls, etc.  The first time this happened, it was funny, but she has been doing this for several years now. Quite frankly, now it comes across as mean.  I was laid off in 2009 and I need a job, but I don't know if I would want to work for her.  You would be constantly looking over your shoulder (and her guests must feel the same way.)  The viewer sees the giant spider sneaking up behind an unsuspecting guests and groans.

A few years ago, Ellen created a Candid Camera type bit using Dennis Quaid.  She would send him out with a ear piece and tell him to say outlandish things to passersby.  Most of his victims took it with good grace, but I always cringed.  This type of humor is similar to April Fool's pranks.  They are funny for the viewer and for the pranksters, but not for the person being pranked.  So again, initially it was funny, but after the second or third time, it grates.  You start sympathizing with the poor guy who hasn't clue why a celebrity is acting like a loon.

SNL used to have a handle on this. As with the samurai, they knew you had to change the joke up to keep it fresh and funny, but they too have experienced overkill.  Remember "The Whiners?"  The skit was hilarious the first time, but after that you wanted to slit your wrists.  Will Ferrell and Cheri Oteri's cheerleaders wore thin due to over use.  What initially was funny and creative became boring and repetitious.

How do you feel?  Am I right?  When is a joke no longer funny?