(CNN) -- When "The Simpsons" recently reached its 500th episode, it did something no TV show had accomplished in 40 years.
And, according to experts on TV history, it may never happen again.
Crashing the 500-episode barrier, which "The Simpsons" did last month, has only been done twice before by a scripted series in prime time. "Gunsmoke," the all-time leader, had 635 episodes before it headed out of Dodge City in 1975. "Lassie" saved the day in 588 telecasts before the courageous collie called it quits in 1973.
"Back when 'Gunsmoke' was on the air, they made 39 episodes a season, almost twice what a regular season is now" said Robert Thompson, professor of television and popular culture at Syracuse University.
In recent decades, a full season order for a network is 22 episodes. On occasion, a network may stretch it to 23 or 24 episodes. "Gunsmoke" remained on the air for 20 seasons but that wouldn't be nearly enough now to generate 500 episodes.
"The Simpsons" hit 500 in the show's 23rd season. Fox has ordered two more seasons, through 2013-14, which will guarantee at least 559 episodes. And after that?
"I don't know where the end is," Al Jean, executive producer, recently told journalists on a conference call. 'I've jokingly said, 'Why not 1,000? Why not 2,000?' But that sounds as preposterous to me now as 500 did then, so I really don't know."
Before the latest renewal, there were reports of protracted negotiations with actors who supplied the voices on the series. Jean said that, to keep the show going, he was required to reduce the overall production cost. Much to his relief, the voice actors went along.
"I personally wouldn't want to do the show without the people that we have," he said. "Had they not signed, we would have stopped the show."
Thompson, of Syracuse, has a hunch that the series will end after the 25th season, well short of the 29 seasons needed to eclipse "Gunsmoke."
"I know some of the producers said it could go on forever," Thompson said. "As an animated show, it probably could. But if I had to make a prediction it would be that after 25 seasons, they're going to call it a day. Its value as a syndicated series is already established."
The two shows being made with the next highest number of episodes are "Law & Order: SVU," which started in 1999 and will have 295 episodes at the end of the season, and "CSI: Crime Scene Investigation," which started in 2000 and will close the season with 273 episodes.
"I don't know if any current series will last long enough" to reach 500 episodes, said Don McGill, a "CSI" executive producer."CSI is in its 12th season and we are having a great time and enjoying it. We are doing well in the ratings.
"We don't personally see an end to 'CSI' now but whether we could go 10 more years to reach 500, I don't know if that's possible. It's a pretty high mark to hit." But, he quickly added, "everybody here is ready to do it."
McGill said that, for a show to last, it has to start with a great idea and be blessed with a great cast and a committed production team. "That's the chemistry you need for a series to have any chance to go to that magic number."
Other things help, too. "I think the fact that the characters don't age is key," Jean said. "I think if Bart was really 40 and living on his parents' couch, it would be too sad."
There is another way of keeping a scripted show fresh, suggested Thompson--"Keep changing the cast."
Characters came and went on "Law & Order," which made 455 episodes before the last "ka-ching" sounded in 2010.
"I think that's a reason why those 'Law & Order' franchises ('Law & Order: SVU' and 'Law & Order: Criminal Intent') could go so long. They'd wear out cast and then get refreshed," Thompson said.
At the same time, there is a trend toward smaller production orders.
A typical season for a scripted show on cable is 13 episodes, Thompson pointed out. "I've got to think that network television is going to move into that cable model to deal with the high cost of production, making it even harder to get to 500."